diff --git a/include/date/date.h b/include/date/date.h
index d2b7bf7..4eb1c3d 100644
--- a/include/date/date.h
+++ b/include/date/date.h
@@ -7,6 +7,7 @@
 // Copyright (c) 2016 Adrian Colomitchi
 // Copyright (c) 2017 Florian Dang
 // Copyright (c) 2017 Paul Thompson
+// Copyright (c) 2018 Tomasz Kamiński
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
 // of this software and associated documentation files (the "Software"), to deal
@@ -95,12 +96,14 @@ namespace date
 #    define CONSTCD11
 #    define CONSTCD14
 #    define NOEXCEPT _NOEXCEPT
+#    define NOEXCEPT_COND(...)
 #  else
 //   VS2017 and later
 #    define CONSTDATA constexpr const
 #    define CONSTCD11 constexpr
 #    define CONSTCD14 constexpr
 #    define NOEXCEPT noexcept
+#    define NOEXCEPT_COND(...) noexcept(__VA_ARGS__)
 #  endif
 
 #elif defined(__SUNPRO_CC) && __SUNPRO_CC <= 0x5150
@@ -109,6 +112,7 @@ namespace date
 #  define CONSTCD11 constexpr
 #  define CONSTCD14
 #  define NOEXCEPT noexcept
+#  define NOEXCEPT_COND(...) noexcept(__VA_ARGS__)
 
 #elif __cplusplus >= 201402
 // C++14
@@ -116,12 +120,14 @@ namespace date
 #  define CONSTCD11 constexpr
 #  define CONSTCD14 constexpr
 #  define NOEXCEPT noexcept
+#  define NOEXCEPT_COND(...) noexcept(__VA_ARGS__)
 #else
 // C++11
 #  define CONSTDATA constexpr const
 #  define CONSTCD11 constexpr
 #  define CONSTCD14
 #  define NOEXCEPT noexcept
+#  define NOEXCEPT_COND(...) noexcept(__VA_ARGS__)
 #endif
 
 #ifndef HAS_VOID_T
@@ -490,6 +496,11 @@ template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
 operator<<(std::basic_ostream<CharT, Traits>& os, const weekday_last& wdl);
 
+#define NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS                                  \
+  NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value     \
+                || (!std::is_convertible<Duration const&, years>::value          \
+                && std::is_nothrow_constructible<months, Duration const&>::value))
+
 // year_month
 
 class year_month
@@ -504,10 +515,19 @@ public:
     CONSTCD11 date::year  year()  const NOEXCEPT;
     CONSTCD11 date::month month() const NOEXCEPT;
 
-    CONSTCD14 year_month& operator+=(const months& dm) NOEXCEPT;
-    CONSTCD14 year_month& operator-=(const months& dm) NOEXCEPT;
-    CONSTCD14 year_month& operator+=(const years& dy) NOEXCEPT;
-    CONSTCD14 year_month& operator-=(const years& dy) NOEXCEPT;
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month& operator+=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month& operator-=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
     CONSTCD11 bool ok() const NOEXCEPT;
 };
@@ -524,9 +544,27 @@ CONSTCD14 year_month operator+(const months& dm, const year_month& ym) NOEXCEPT;
 CONSTCD14 year_month operator-(const year_month& ym, const months& dm) NOEXCEPT;
 
 CONSTCD11 months operator-(const year_month& x, const year_month& y) NOEXCEPT;
-CONSTCD11 year_month operator+(const year_month& ym, const years& dy) NOEXCEPT;
-CONSTCD11 year_month operator+(const years& dy, const year_month& ym) NOEXCEPT;
-CONSTCD11 year_month operator-(const year_month& ym, const years& dy) NOEXCEPT;
+
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, years>::value
+          >::type>
+CONSTCD11 year_month operator+(const year_month& ym, const Duration& d)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value);
+
+template <class Duration,
+          class = typename std::enable_if<
+           std::is_convertible<Duration const&, years>::value
+          >::type>
+CONSTCD11 year_month operator+(const Duration& d, const year_month& ym)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value);
+
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, years>::value
+          >::type>
+CONSTCD11 year_month operator-(const year_month& ym, const Duration& d)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value);
 
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
@@ -650,10 +688,19 @@ public:
     CONSTCD14 year_month_day(sys_days dp) NOEXCEPT;
     CONSTCD14 explicit year_month_day(local_days dp) NOEXCEPT;
 
-    CONSTCD14 year_month_day& operator+=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_day& operator-=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_day& operator+=(const years& y)  NOEXCEPT;
-    CONSTCD14 year_month_day& operator-=(const years& y)  NOEXCEPT;
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_day& operator+=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_day& operator-=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
     CONSTCD11 date::year  year()  const NOEXCEPT;
     CONSTCD11 date::month month() const NOEXCEPT;
@@ -675,12 +722,29 @@ CONSTCD11 bool operator> (const year_month_day& x, const year_month_day& y) NOEX
 CONSTCD11 bool operator<=(const year_month_day& x, const year_month_day& y) NOEXCEPT;
 CONSTCD11 bool operator>=(const year_month_day& x, const year_month_day& y) NOEXCEPT;
 
-CONSTCD14 year_month_day operator+(const year_month_day& ymd, const months& dm) NOEXCEPT;
-CONSTCD14 year_month_day operator+(const months& dm, const year_month_day& ymd) NOEXCEPT;
-CONSTCD14 year_month_day operator-(const year_month_day& ymd, const months& dm) NOEXCEPT;
-CONSTCD11 year_month_day operator+(const year_month_day& ymd, const years& dy)  NOEXCEPT;
-CONSTCD11 year_month_day operator+(const years& dy, const year_month_day& ymd)  NOEXCEPT;
-CONSTCD11 year_month_day operator-(const year_month_day& ymd, const years& dy)  NOEXCEPT;
+
+CONSTCD11 year_month operator/(const year& y, const month& m) NOEXCEPT;
+
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_day operator+(const year_month_day& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_day operator+(const Duration& d, const year_month_day& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_day operator-(const year_month_day& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
@@ -697,10 +761,19 @@ public:
     CONSTCD11 year_month_day_last(const date::year& y,
                                   const date::month_day_last& mdl) NOEXCEPT;
 
-    CONSTCD14 year_month_day_last& operator+=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_day_last& operator-=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_day_last& operator+=(const years& y)  NOEXCEPT;
-    CONSTCD14 year_month_day_last& operator-=(const years& y)  NOEXCEPT;
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_day_last& operator+=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_day_last& operator-=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
     CONSTCD11 date::year           year()           const NOEXCEPT;
     CONSTCD11 date::month          month()          const NOEXCEPT;
@@ -725,29 +798,29 @@ CONSTCD11
 CONSTCD11
     bool operator>=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
 
-CONSTCD14
-year_month_day_last
-operator+(const year_month_day_last& ymdl, const months& dm) NOEXCEPT;
-
-CONSTCD14
-year_month_day_last
-operator+(const months& dm, const year_month_day_last& ymdl) NOEXCEPT;
-
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_day_last
-operator+(const year_month_day_last& ymdl, const years& dy) NOEXCEPT;
+    year_month_day_last operator+(const year_month_day_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_day_last
-operator+(const years& dy, const year_month_day_last& ymdl) NOEXCEPT;
-
-CONSTCD14
-year_month_day_last
-operator-(const year_month_day_last& ymdl, const months& dm) NOEXCEPT;
+    year_month_day_last operator+(const Duration& d, const year_month_day_last& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_day_last
-operator-(const year_month_day_last& ymdl, const years& dy) NOEXCEPT;
+    year_month_day_last operator-(const year_month_day_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
@@ -768,10 +841,19 @@ public:
     CONSTCD14 year_month_weekday(const sys_days& dp) NOEXCEPT;
     CONSTCD14 explicit year_month_weekday(const local_days& dp) NOEXCEPT;
 
-    CONSTCD14 year_month_weekday& operator+=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_weekday& operator-=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_weekday& operator+=(const years& y)  NOEXCEPT;
-    CONSTCD14 year_month_weekday& operator-=(const years& y)  NOEXCEPT;
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_weekday& operator+=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_weekday& operator-=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
     CONSTCD11 date::year year() const NOEXCEPT;
     CONSTCD11 date::month month() const NOEXCEPT;
@@ -793,29 +875,26 @@ CONSTCD11
 CONSTCD11
     bool operator!=(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT;
 
-CONSTCD14
-year_month_weekday
-operator+(const year_month_weekday& ymwd, const months& dm) NOEXCEPT;
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_weekday operator+(const year_month_weekday& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
-CONSTCD14
-year_month_weekday
-operator+(const months& dm, const year_month_weekday& ymwd) NOEXCEPT;
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_weekday operator+(const Duration& d, const year_month_weekday& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
-CONSTCD11
-year_month_weekday
-operator+(const year_month_weekday& ymwd, const years& dy) NOEXCEPT;
-
-CONSTCD11
-year_month_weekday
-operator+(const years& dy, const year_month_weekday& ymwd) NOEXCEPT;
-
-CONSTCD14
-year_month_weekday
-operator-(const year_month_weekday& ymwd, const months& dm) NOEXCEPT;
-
-CONSTCD11
-year_month_weekday
-operator-(const year_month_weekday& ymwd, const years& dy) NOEXCEPT;
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
+CONSTCD11 year_month_weekday operator-(const year_month_weekday& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
@@ -833,10 +912,19 @@ public:
     CONSTCD11 year_month_weekday_last(const date::year& y, const date::month& m,
                                       const date::weekday_last& wdl) NOEXCEPT;
 
-    CONSTCD14 year_month_weekday_last& operator+=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_weekday_last& operator-=(const months& m) NOEXCEPT;
-    CONSTCD14 year_month_weekday_last& operator+=(const years& y) NOEXCEPT;
-    CONSTCD14 year_month_weekday_last& operator-=(const years& y) NOEXCEPT;
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_weekday_last& operator+=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
+
+    template <class Duration,
+              class = typename std::enable_if<
+                std::is_convertible<Duration const&, months>::value
+              >::type>
+    CONSTCD14 year_month_weekday_last& operator-=(const Duration& d)
+    NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
     CONSTCD11 date::year year() const NOEXCEPT;
     CONSTCD11 date::month month() const NOEXCEPT;
@@ -859,29 +947,29 @@ CONSTCD11
 bool
 operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) NOEXCEPT;
 
-CONSTCD14
-year_month_weekday_last
-operator+(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT;
-
-CONSTCD14
-year_month_weekday_last
-operator+(const months& dm, const year_month_weekday_last& ymwdl) NOEXCEPT;
-
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_weekday_last
-operator+(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT;
+    year_month_weekday_last operator+(const year_month_weekday_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_weekday_last
-operator+(const years& dy, const year_month_weekday_last& ymwdl) NOEXCEPT;
-
-CONSTCD14
-year_month_weekday_last
-operator-(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT;
+    year_month_weekday_last operator+(const Duration& d, const year_month_weekday_last& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
+template <class Duration,
+          class = typename std::enable_if<
+            std::is_convertible<Duration const&, months>::value
+          >::type>
 CONSTCD11
-year_month_weekday_last
-operator-(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT;
+    year_month_weekday_last operator-(const year_month_weekday_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS;
 
 template<class CharT, class Traits>
 std::basic_ostream<CharT, Traits>&
@@ -1982,39 +2070,25 @@ CONSTCD11 inline year year_month::year() const NOEXCEPT {return y_;}
 CONSTCD11 inline month year_month::month() const NOEXCEPT {return m_;}
 CONSTCD11 inline bool year_month::ok() const NOEXCEPT {return y_.ok() && m_.ok();}
 
+template <class Duration, class>
 CONSTCD14
 inline
 year_month&
-year_month::operator+=(const months& dm) NOEXCEPT
+year_month::operator+=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
 {
-    *this = *this + dm;
+    *this = *this + d;
     return *this;
 }
 
+template <class Duration, class>
 CONSTCD14
 inline
 year_month&
-year_month::operator-=(const months& dm) NOEXCEPT
+year_month::operator-=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
 {
-    *this = *this - dm;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month&
-year_month::operator+=(const years& dy) NOEXCEPT
-{
-    *this = *this + dy;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month&
-year_month::operator-=(const years& dy) NOEXCEPT
-{
-    *this = *this - dy;
+    *this = *this - d;
     return *this;
 }
 
@@ -2104,28 +2178,34 @@ operator-(const year_month& x, const year_month& y) NOEXCEPT
             months(static_cast<unsigned>(x.month()) - static_cast<unsigned>(y.month()));
 }
 
+template <class Duration, class>
 CONSTCD11
 inline
 year_month
-operator+(const year_month& ym, const years& dy) NOEXCEPT
+operator+(const year_month& ym, const Duration& d)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value)
 {
-    return (ym.year() + dy) / ym.month();
+    return (ym.year() + years(d)) / ym.month();;
 }
 
+template <class Duration, class>
 CONSTCD11
 inline
 year_month
-operator+(const years& dy, const year_month& ym) NOEXCEPT
+operator+(const Duration& d, const year_month& ym)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value)
 {
-    return ym + dy;
+    return ym + years(d);
 }
 
+template <class Duration, class>
 CONSTCD11
 inline
 year_month
-operator-(const year_month& ym, const years& dy) NOEXCEPT
+operator-(const year_month& ym, const Duration& d)
+NOEXCEPT_COND(std::is_nothrow_constructible<years, Duration const&>::value)
 {
-    return ym + -dy;
+   return ym + -years(d);
 }
 
 template<class CharT, class Traits>
@@ -2397,42 +2477,6 @@ year_month_day_last::year_month_day_last(const date::year& y,
     , mdl_(mdl)
     {}
 
-CONSTCD14
-inline
-year_month_day_last&
-year_month_day_last::operator+=(const months& m) NOEXCEPT
-{
-    *this = *this + m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_day_last&
-year_month_day_last::operator-=(const months& m) NOEXCEPT
-{
-    *this = *this - m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_day_last&
-year_month_day_last::operator+=(const years& y) NOEXCEPT
-{
-    *this = *this + y;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_day_last&
-year_month_day_last::operator-=(const years& y) NOEXCEPT
-{
-    *this = *this - y;
-    return *this;
-}
-
 CONSTCD11 inline year year_month_day_last::year() const NOEXCEPT {return y_;}
 CONSTCD11 inline month year_month_day_last::month() const NOEXCEPT {return mdl_.month();}
 
@@ -2460,6 +2504,28 @@ year_month_day_last::day() const NOEXCEPT
         d[static_cast<unsigned>(month()) - 1] : date::day{29};
 }
 
+template <class Duration, class>
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator+=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this + d;
+    return *this;
+}
+
+template <class Duration, class>
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator-=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this - d;
+    return *this;
+}
+
 CONSTCD14
 inline
 year_month_day_last::operator sys_days() const NOEXCEPT
@@ -2532,6 +2598,36 @@ operator>=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
     return !(x < y);
 }
 
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day_last
+operator+(const year_month_day_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / last_spec();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day_last
+operator+(const Duration& d, const year_month_day_last& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / last_spec();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day_last
+operator-(const year_month_day_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() - d) / last_spec();
+}
+
 template<class CharT, class Traits>
 inline
 std::basic_ostream<CharT, Traits>&
@@ -2540,54 +2636,6 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_day_last& ymd
     return os << ymdl.year() << '/' << ymdl.month_day_last();
 }
 
-CONSTCD14
-inline
-year_month_day_last
-operator+(const year_month_day_last& ymdl, const months& dm) NOEXCEPT
-{
-    return (ymdl.year() / ymdl.month() + dm) / last;
-}
-
-CONSTCD14
-inline
-year_month_day_last
-operator+(const months& dm, const year_month_day_last& ymdl) NOEXCEPT
-{
-    return ymdl + dm;
-}
-
-CONSTCD14
-inline
-year_month_day_last
-operator-(const year_month_day_last& ymdl, const months& dm) NOEXCEPT
-{
-    return ymdl + (-dm);
-}
-
-CONSTCD11
-inline
-year_month_day_last
-operator+(const year_month_day_last& ymdl, const years& dy) NOEXCEPT
-{
-    return {ymdl.year()+dy, ymdl.month_day_last()};
-}
-
-CONSTCD11
-inline
-year_month_day_last
-operator+(const years& dy, const year_month_day_last& ymdl) NOEXCEPT
-{
-    return ymdl + dy;
-}
-
-CONSTCD11
-inline
-year_month_day_last
-operator-(const year_month_day_last& ymdl, const years& dy) NOEXCEPT
-{
-    return ymdl + (-dy);
-}
-
 // year_month_day
 
 CONSTCD11
@@ -2623,39 +2671,25 @@ CONSTCD11 inline year year_month_day::year() const NOEXCEPT {return y_;}
 CONSTCD11 inline month year_month_day::month() const NOEXCEPT {return m_;}
 CONSTCD11 inline day year_month_day::day() const NOEXCEPT {return d_;}
 
+template <class Duration, class>
 CONSTCD14
 inline
 year_month_day&
-year_month_day::operator+=(const months& m) NOEXCEPT
+year_month_day::operator+=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
 {
-    *this = *this + m;
+    *this = *this + d;
     return *this;
 }
 
+template <class Duration, class>
 CONSTCD14
 inline
 year_month_day&
-year_month_day::operator-=(const months& m) NOEXCEPT
+year_month_day::operator-=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
 {
-    *this = *this - m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_day&
-year_month_day::operator+=(const years& y) NOEXCEPT
-{
-    *this = *this + y;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_day&
-year_month_day::operator-=(const years& y) NOEXCEPT
-{
-    *this = *this - y;
+    *this = *this - d;
     return *this;
 }
 
@@ -2754,6 +2788,36 @@ operator>=(const year_month_day& x, const year_month_day& y) NOEXCEPT
     return !(x < y);
 }
 
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day
+operator+(const year_month_day& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.day();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day
+operator+(const Duration& d, const year_month_day& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.day();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_day
+operator-(const year_month_day& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() - d) / ymd.day();
+}
+
 template<class CharT, class Traits>
 inline
 std::basic_ostream<CharT, Traits>&
@@ -2792,54 +2856,6 @@ year_month_day::from_days(days dp) NOEXCEPT
     return year_month_day{date::year{y + (m <= 2)}, date::month(m), date::day(d)};
 }
 
-CONSTCD14
-inline
-year_month_day
-operator+(const year_month_day& ymd, const months& dm) NOEXCEPT
-{
-    return (ymd.year() / ymd.month() + dm) / ymd.day();
-}
-
-CONSTCD14
-inline
-year_month_day
-operator+(const months& dm, const year_month_day& ymd) NOEXCEPT
-{
-    return ymd + dm;
-}
-
-CONSTCD14
-inline
-year_month_day
-operator-(const year_month_day& ymd, const months& dm) NOEXCEPT
-{
-    return ymd + (-dm);
-}
-
-CONSTCD11
-inline
-year_month_day
-operator+(const year_month_day& ymd, const years& dy) NOEXCEPT
-{
-    return (ymd.year() + dy) / ymd.month() / ymd.day();
-}
-
-CONSTCD11
-inline
-year_month_day
-operator+(const years& dy, const year_month_day& ymd) NOEXCEPT
-{
-    return ymd + dy;
-}
-
-CONSTCD11
-inline
-year_month_day
-operator-(const year_month_day& ymd, const years& dy) NOEXCEPT
-{
-    return ymd + (-dy);
-}
-
 // year_month_weekday
 
 CONSTCD11
@@ -2864,42 +2880,6 @@ year_month_weekday::year_month_weekday(const local_days& dp) NOEXCEPT
     : year_month_weekday(from_days(dp.time_since_epoch()))
     {}
 
-CONSTCD14
-inline
-year_month_weekday&
-year_month_weekday::operator+=(const months& m) NOEXCEPT
-{
-    *this = *this + m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday&
-year_month_weekday::operator-=(const months& m) NOEXCEPT
-{
-    *this = *this - m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday&
-year_month_weekday::operator+=(const years& y) NOEXCEPT
-{
-    *this = *this + y;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday&
-year_month_weekday::operator-=(const years& y) NOEXCEPT
-{
-    *this = *this - y;
-    return *this;
-}
-
 CONSTCD11 inline year year_month_weekday::year() const NOEXCEPT {return y_;}
 CONSTCD11 inline month year_month_weekday::month() const NOEXCEPT {return m_;}
 
@@ -2927,6 +2907,26 @@ year_month_weekday::weekday_indexed() const NOEXCEPT
     return wdi_;
 }
 
+template <class Duration, class>
+CONSTCD14
+year_month_weekday&
+year_month_weekday::operator+=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this + d;
+    return *this;
+}
+
+template <class Duration, class>
+CONSTCD14
+year_month_weekday&
+year_month_weekday::operator-=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this - d;
+    return *this;
+}
+
 CONSTCD14
 inline
 year_month_weekday::operator sys_days() const NOEXCEPT
@@ -2992,6 +2992,36 @@ operator!=(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT
     return !(x == y);
 }
 
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday
+operator+(const year_month_weekday& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.weekday_indexed();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday
+operator+(const Duration& d, const year_month_weekday& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.weekday_indexed();;
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday
+operator-(const year_month_weekday& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() - d) / ymd.weekday_indexed();
+}
+
 template<class CharT, class Traits>
 inline
 std::basic_ostream<CharT, Traits>&
@@ -3001,54 +3031,6 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday& ymwd
               << '/' << ymwdi.weekday_indexed();
 }
 
-CONSTCD14
-inline
-year_month_weekday
-operator+(const year_month_weekday& ymwd, const months& dm) NOEXCEPT
-{
-    return (ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed();
-}
-
-CONSTCD14
-inline
-year_month_weekday
-operator+(const months& dm, const year_month_weekday& ymwd) NOEXCEPT
-{
-    return ymwd + dm;
-}
-
-CONSTCD14
-inline
-year_month_weekday
-operator-(const year_month_weekday& ymwd, const months& dm) NOEXCEPT
-{
-    return ymwd + (-dm);
-}
-
-CONSTCD11
-inline
-year_month_weekday
-operator+(const year_month_weekday& ymwd, const years& dy) NOEXCEPT
-{
-    return {ymwd.year()+dy, ymwd.month(), ymwd.weekday_indexed()};
-}
-
-CONSTCD11
-inline
-year_month_weekday
-operator+(const years& dy, const year_month_weekday& ymwd) NOEXCEPT
-{
-    return ymwd + dy;
-}
-
-CONSTCD11
-inline
-year_month_weekday
-operator-(const year_month_weekday& ymwd, const years& dy) NOEXCEPT
-{
-    return ymwd + (-dy);
-}
-
 // year_month_weekday_last
 
 CONSTCD11
@@ -3061,42 +3043,6 @@ year_month_weekday_last::year_month_weekday_last(const date::year& y,
     , wdl_(wdl)
     {}
 
-CONSTCD14
-inline
-year_month_weekday_last&
-year_month_weekday_last::operator+=(const months& m) NOEXCEPT
-{
-    *this = *this + m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday_last&
-year_month_weekday_last::operator-=(const months& m) NOEXCEPT
-{
-    *this = *this - m;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday_last&
-year_month_weekday_last::operator+=(const years& y) NOEXCEPT
-{
-    *this = *this + y;
-    return *this;
-}
-
-CONSTCD14
-inline
-year_month_weekday_last&
-year_month_weekday_last::operator-=(const years& y) NOEXCEPT
-{
-    *this = *this - y;
-    return *this;
-}
-
 CONSTCD11 inline year year_month_weekday_last::year() const NOEXCEPT {return y_;}
 CONSTCD11 inline month year_month_weekday_last::month() const NOEXCEPT {return m_;}
 
@@ -3116,6 +3062,28 @@ year_month_weekday_last::weekday_last() const NOEXCEPT
     return wdl_;
 }
 
+template <class Duration, class>
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator+=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this + d;
+    return *this;
+}
+
+template <class Duration, class>
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator-=(const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    *this = *this - d;
+    return *this;
+}
+
 CONSTCD14
 inline
 year_month_weekday_last::operator sys_days() const NOEXCEPT
@@ -3164,6 +3132,36 @@ operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) N
     return !(x == y);
 }
 
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday_last
+operator+(const year_month_weekday_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.weekday_last();
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday_last
+operator+(const Duration& d, const year_month_weekday_last& ymd)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() + d) / ymd.weekday_last();;
+}
+
+template <class Duration, class>
+CONSTCD11
+inline
+year_month_weekday_last
+operator-(const year_month_weekday_last& ymd, const Duration& d)
+NOEXCEPT_CONVERTIBLE_TO_YEARS_OR_MONTHS
+{
+    return (ymd.year() / ymd.month() - d) / ymd.weekday_last();
+}
+
 template<class CharT, class Traits>
 inline
 std::basic_ostream<CharT, Traits>&
@@ -3172,54 +3170,6 @@ operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday_last&
     return os << ymwdl.year() << '/' << ymwdl.month() << '/' << ymwdl.weekday_last();
 }
 
-CONSTCD14
-inline
-year_month_weekday_last
-operator+(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT
-{
-    return (ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last();
-}
-
-CONSTCD14
-inline
-year_month_weekday_last
-operator+(const months& dm, const year_month_weekday_last& ymwdl) NOEXCEPT
-{
-    return ymwdl + dm;
-}
-
-CONSTCD14
-inline
-year_month_weekday_last
-operator-(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT
-{
-    return ymwdl + (-dm);
-}
-
-CONSTCD11
-inline
-year_month_weekday_last
-operator+(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT
-{
-    return {ymwdl.year()+dy, ymwdl.month(), ymwdl.weekday_last()};
-}
-
-CONSTCD11
-inline
-year_month_weekday_last
-operator+(const years& dy, const year_month_weekday_last& ymwdl) NOEXCEPT
-{
-    return ymwdl + dy;
-}
-
-CONSTCD11
-inline
-year_month_weekday_last
-operator-(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT
-{
-    return ymwdl + (-dy);
-}
-
 // year_month from operator/()
 
 CONSTCD11
diff --git a/test/clock_cast_test/custom_clock.pass.cpp b/test/clock_cast_test/custom_clock.pass.cpp
index d9c9d8b..8d3ccba 100644
--- a/test/clock_cast_test/custom_clock.pass.cpp
+++ b/test/clock_cast_test/custom_clock.pass.cpp
@@ -30,16 +30,16 @@ int conversions = 0;
 //to/from impl
 struct mil_clock
 {
-  using duration = std::common_type_t<std::chrono::system_clock::duration, date::days>;
+  using duration = typename std::common_type<std::chrono::system_clock::duration, date::days>::type;
   using rep = duration::rep;
   using period = duration::period;
   using time_point = std::chrono::time_point<mil_clock, duration>;
 
-  static constexpr date::sys_days epoch = date::year{2000}/date::month{0}/date::day{1};
+  static constexpr date::sys_days epoch{date::days{1000}};
 
   template<typename Duration>
   static
-  std::chrono::time_point<std::chrono::system_clock, std::common_type_t<Duration, date::days>>
+  std::chrono::time_point<std::chrono::system_clock, typename std::common_type<Duration, date::days>::type>
   to_sys(std::chrono::time_point<mil_clock, Duration> const& tp)
   {
     ++conversions;
@@ -48,11 +48,11 @@ struct mil_clock
 
   template<typename Duration>
   static
-  std::chrono::time_point<mil_clock, std::common_type_t<Duration, date::days>>
+  std::chrono::time_point<mil_clock, typename std::common_type<Duration, date::days>::type>
   from_sys(std::chrono::time_point<std::chrono::system_clock, Duration> const& tp)
   {
     ++conversions;
-    using res = std::chrono::time_point<mil_clock, std::common_type_t<Duration, date::days>>;
+    using res = std::chrono::time_point<mil_clock, typename std::common_type<Duration, date::days>::type>;
     return res(tp - epoch);
   }
 
@@ -103,11 +103,11 @@ namespace date
    struct clock_time_conversion<mil_clock, s2s_clock>
    {
      template<typename Duration>
-     std::chrono::time_point<mil_clock, std::common_type_t<Duration, date::days>>
+     std::chrono::time_point<mil_clock, typename std::common_type<Duration, date::days>::type>
      operator()(std::chrono::time_point<s2s_clock, Duration> const& tp)
      {
        ++conversions;
-       using res = std::chrono::time_point<mil_clock, std::common_type_t<Duration, date::days>>;
+       using res = std::chrono::time_point<mil_clock, typename std::common_type<Duration, date::days>::type>;
        return res(tp.time_since_epoch() - mil_clock::epoch.time_since_epoch());
      }
    };
diff --git a/test/clock_cast_test/to_sys_return_utc_time.fail.cpp b/test/clock_cast_test/to_sys_return_utc_time.fail.cpp
index d5364ce..6fc7a2b 100644
--- a/test/clock_cast_test/to_sys_return_utc_time.fail.cpp
+++ b/test/clock_cast_test/to_sys_return_utc_time.fail.cpp
@@ -34,7 +34,7 @@ struct bad_clock
   date::utc_time<Duration>
   to_sys(std::chrono::time_point<bad_clock, Duration> const& tp)
   {
-    return utc_time<Duration>(tp.time_since_epoch());
+    return date::utc_time<Duration>(tp.time_since_epoch());
   }
 };
 
diff --git a/test/date_test/multi_year_duration_addition.pass.cpp b/test/date_test/multi_year_duration_addition.pass.cpp
new file mode 100644
index 0000000..0de3479
--- /dev/null
+++ b/test/date_test/multi_year_duration_addition.pass.cpp
@@ -0,0 +1,297 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2018 Tomasz Kamiński
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "date.h"
+#include <chrono>
+#include <cassert>
+#include <type_traits>
+
+#define CPP11_ASSERT(...) static_assert(__VA_ARGS__, "")
+
+#if __cplusplus >= 201402
+// C++14
+#  define CPP14_ASSERT(...) static_assert(__VA_ARGS__, "")
+#else
+// C++11
+#  define CPP14_ASSERT(...) assert(__VA_ARGS__)
+#endif
+
+#define NOEXCEPT_ASSERT(...) static_assert(noexcept(__VA_ARGS__), "")
+
+//Invocation involves a conversion between duration that is currently
+//not marked as noexcept.
+#define NOEXCEPT_CONVERSION(...)
+
+template<typename T>
+constexpr T copy(T const& t) noexcept { return t; }
+
+
+int
+main()
+{
+    using namespace date;
+    using namespace std::chrono;
+
+    using decades = duration<int, std::ratio_multiply<std::ratio<10>, years::period>>;
+    using decamonths = duration<int, std::ratio_multiply<std::ratio<10>, months::period>>;
+
+    constexpr months one_month{1};
+    constexpr years one_year{1};
+    constexpr decades one_decade{1};
+    constexpr decamonths one_decamonth{1};
+
+    {
+       constexpr year_month ym = 2001_y/feb;
+       CPP14_ASSERT(ym + one_month == 2001_y/mar);
+       NOEXCEPT_ASSERT(ym + one_month);
+       CPP14_ASSERT(one_month + ym == 2001_y/mar);
+       NOEXCEPT_ASSERT(one_month + ym);
+       CPP14_ASSERT(ym - one_month == 2001_y/jan);
+       NOEXCEPT_ASSERT(ym - one_month);
+       CPP14_ASSERT((copy(ym) += one_month) == 2001_y/mar);
+       NOEXCEPT_ASSERT(copy(ym) += one_month);
+       CPP14_ASSERT((copy(ym) -= one_month) == 2001_y/jan);
+       NOEXCEPT_ASSERT(copy(ym) -= one_month);
+      
+       CPP11_ASSERT(ym + one_year == 2002_y/feb);
+       NOEXCEPT_ASSERT(ym + one_year);
+       CPP11_ASSERT(one_year + ym == 2002_y/feb);
+       NOEXCEPT_ASSERT(one_year + ym);
+       CPP11_ASSERT(ym - one_year == 2000_y/feb);
+       NOEXCEPT_ASSERT(ym - one_year);
+       CPP14_ASSERT((copy(ym) += one_year) == 2002_y/feb);
+       NOEXCEPT_ASSERT(copy(ym) += one_year);
+       CPP14_ASSERT((copy(ym) -= one_year) == 2000_y/feb);
+       NOEXCEPT_ASSERT(copy(ym) -= one_year);
+
+       CPP11_ASSERT(ym + one_decade == 2011_y/feb);
+       NOEXCEPT_CONVERSION(ym + one_decade);
+       CPP11_ASSERT(one_decade + ym == 2011_y/feb);
+       NOEXCEPT_CONVERSION(one_decade + ym);
+       CPP11_ASSERT(ym - one_decade == 1991_y/feb);
+       NOEXCEPT_CONVERSION(ym - one_decade);
+       CPP14_ASSERT((copy(ym) += one_decade) == 2011_y/feb);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decade);
+       CPP14_ASSERT((copy(ym) -= one_decade) == 1991_y/feb);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decade);
+
+       CPP14_ASSERT(ym + one_decamonth == 2001_y/dec);
+       NOEXCEPT_CONVERSION(ym + one_decamonth);
+       CPP14_ASSERT(one_decamonth + ym  == 2001_y/dec);
+       NOEXCEPT_CONVERSION(one_decamonth + ym);
+       CPP14_ASSERT(ym - one_decamonth == 2000_y/apr);
+       NOEXCEPT_CONVERSION(ym - one_decamonth);
+       CPP14_ASSERT((copy(ym) += one_decamonth) == 2001_y/dec);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decamonth);
+       CPP14_ASSERT((copy(ym) -= one_decamonth) == 2000_y/apr);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decamonth);
+    }
+
+    {
+       constexpr year_month_day ym = 2001_y/feb/10;
+       CPP14_ASSERT(ym + one_month == 2001_y/mar/10);
+       NOEXCEPT_ASSERT(ym + one_month);
+       CPP14_ASSERT(one_month + ym == 2001_y/mar/10);
+       NOEXCEPT_ASSERT(one_month + ym);
+       CPP14_ASSERT(ym - one_month == 2001_y/jan/10);
+       NOEXCEPT_ASSERT(ym - one_month);
+       CPP14_ASSERT((copy(ym) += one_month) == 2001_y/mar/10);
+       NOEXCEPT_ASSERT(copy(ym) += one_month);
+       CPP14_ASSERT((copy(ym) -= one_month) == 2001_y/jan/10);
+       NOEXCEPT_ASSERT(copy(ym) -= one_month);
+      
+       CPP11_ASSERT(ym + one_year == 2002_y/feb/10);
+       NOEXCEPT_ASSERT(ym + one_year);
+       CPP11_ASSERT(one_year + ym == 2002_y/feb/10);
+       NOEXCEPT_ASSERT(one_year + ym);
+       CPP11_ASSERT(ym - one_year == 2000_y/feb/10);
+       NOEXCEPT_ASSERT(ym - one_year);
+       CPP14_ASSERT((copy(ym) += one_year) == 2002_y/feb/10);
+       NOEXCEPT_ASSERT(copy(ym) += one_year);
+       CPP14_ASSERT((copy(ym) -= one_year) == 2000_y/feb/10);
+       NOEXCEPT_ASSERT(copy(ym) -= one_year);
+
+       CPP11_ASSERT(ym + one_decade == 2011_y/feb/10);
+       NOEXCEPT_CONVERSION(ym + one_decade);
+       CPP11_ASSERT(one_decade + ym == 2011_y/feb/10);
+       NOEXCEPT_CONVERSION(one_decade + ym);
+       CPP11_ASSERT(ym - one_decade == 1991_y/feb/10);
+       NOEXCEPT_CONVERSION(ym - one_decade);
+       CPP14_ASSERT((copy(ym) += one_decade) == 2011_y/feb/10);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decade);
+       CPP14_ASSERT((copy(ym) -= one_decade) == 1991_y/feb/10);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decade);
+
+       CPP14_ASSERT(ym + one_decamonth == 2001_y/dec/10);
+       NOEXCEPT_CONVERSION(ym + one_decamonth);
+       CPP14_ASSERT(one_decamonth + ym  == 2001_y/dec/10);
+       NOEXCEPT_CONVERSION(one_decamonth + ym);
+       CPP14_ASSERT(ym - one_decamonth == 2000_y/apr/10);
+       NOEXCEPT_CONVERSION(ym - one_decamonth);
+       CPP14_ASSERT((copy(ym) += one_decamonth) == 2001_y/dec/10);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decamonth);
+       CPP14_ASSERT((copy(ym) -= one_decamonth) == 2000_y/apr/10);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decamonth);
+    }
+
+    {
+       constexpr year_month_day_last ym = 2001_y/feb/last;
+       CPP14_ASSERT(ym + one_month == 2001_y/mar/last);
+       NOEXCEPT_ASSERT(ym + one_month);
+       CPP14_ASSERT(one_month + ym == 2001_y/mar/last);
+       NOEXCEPT_ASSERT(one_month + ym);
+       CPP14_ASSERT(ym - one_month == 2001_y/jan/last);
+       NOEXCEPT_ASSERT(ym - one_month);
+       CPP14_ASSERT((copy(ym) += one_month) == 2001_y/mar/last);
+       NOEXCEPT_ASSERT(copy(ym) += one_month);
+       CPP14_ASSERT((copy(ym) -= one_month) == 2001_y/jan/last);
+       NOEXCEPT_ASSERT(copy(ym) -= one_month);
+      
+       CPP11_ASSERT(ym + one_year == 2002_y/feb/last);
+       NOEXCEPT_ASSERT(ym + one_year);
+       CPP11_ASSERT(one_year + ym == 2002_y/feb/last);
+       NOEXCEPT_ASSERT(one_year + ym);
+       CPP11_ASSERT(ym - one_year == 2000_y/feb/last);
+       NOEXCEPT_ASSERT(ym - one_year);
+       CPP14_ASSERT((copy(ym) += one_year) == 2002_y/feb/last);
+       NOEXCEPT_ASSERT(copy(ym) += one_year);
+       CPP14_ASSERT((copy(ym) -= one_year) == 2000_y/feb/last);
+       NOEXCEPT_ASSERT(copy(ym) -= one_year);
+
+       CPP11_ASSERT(ym + one_decade == 2011_y/feb/last);
+       NOEXCEPT_CONVERSION(ym + one_decade);
+       CPP11_ASSERT(one_decade + ym == 2011_y/feb/last);
+       NOEXCEPT_CONVERSION(one_decade + ym);
+       CPP11_ASSERT(ym - one_decade == 1991_y/feb/last);
+       NOEXCEPT_CONVERSION(ym - one_decade);
+       CPP14_ASSERT((copy(ym) += one_decade) == 2011_y/feb/last);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decade);
+       CPP14_ASSERT((copy(ym) -= one_decade) == 1991_y/feb/last);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decade);
+
+       CPP14_ASSERT(ym + one_decamonth == 2001_y/dec/last);
+       NOEXCEPT_CONVERSION(ym + one_decamonth);
+       CPP14_ASSERT(one_decamonth + ym  == 2001_y/dec/last);
+       NOEXCEPT_CONVERSION(one_decamonth + ym);
+       CPP14_ASSERT(ym - one_decamonth == 2000_y/apr/last);
+       NOEXCEPT_CONVERSION(ym - one_decamonth);
+       CPP14_ASSERT((copy(ym) += one_decamonth) == 2001_y/dec/last);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decamonth);
+       CPP14_ASSERT((copy(ym) -= one_decamonth) == 2000_y/apr/last);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decamonth);
+    }
+
+    {
+       constexpr year_month_weekday ym = 2001_y/feb/fri[4];
+       CPP14_ASSERT(ym + one_month == 2001_y/mar/fri[4]);
+       NOEXCEPT_ASSERT(ym + one_month);
+       CPP14_ASSERT(one_month + ym == 2001_y/mar/fri[4]);
+       NOEXCEPT_ASSERT(one_month + ym);
+       CPP14_ASSERT(ym - one_month == 2001_y/jan/fri[4]);
+       NOEXCEPT_ASSERT(ym - one_month);
+       CPP14_ASSERT((copy(ym) += one_month) == 2001_y/mar/fri[4]);
+       NOEXCEPT_ASSERT(copy(ym) += one_month);
+       CPP14_ASSERT((copy(ym) -= one_month) == 2001_y/jan/fri[4]);
+       NOEXCEPT_ASSERT(copy(ym) -= one_month);
+      
+       CPP11_ASSERT(ym + one_year == 2002_y/feb/fri[4]);
+       NOEXCEPT_ASSERT(ym + one_year);
+       CPP11_ASSERT(one_year + ym == 2002_y/feb/fri[4]);
+       NOEXCEPT_ASSERT(one_year + ym);
+       CPP11_ASSERT(ym - one_year == 2000_y/feb/fri[4]);
+       NOEXCEPT_ASSERT(ym - one_year);
+       CPP14_ASSERT((copy(ym) += one_year) == 2002_y/feb/fri[4]);
+       NOEXCEPT_ASSERT(copy(ym) += one_year);
+       CPP14_ASSERT((copy(ym) -= one_year) == 2000_y/feb/fri[4]);
+       NOEXCEPT_ASSERT(copy(ym) -= one_year);
+
+       CPP11_ASSERT(ym + one_decade == 2011_y/feb/fri[4]);
+       NOEXCEPT_CONVERSION(ym + one_decade);
+       CPP11_ASSERT(one_decade + ym == 2011_y/feb/fri[4]);
+       NOEXCEPT_CONVERSION(one_decade + ym);
+       CPP11_ASSERT(ym - one_decade == 1991_y/feb/fri[4]);
+       NOEXCEPT_CONVERSION(ym - one_decade);
+       CPP14_ASSERT((copy(ym) += one_decade) == 2011_y/feb/fri[4]);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decade);
+       CPP14_ASSERT((copy(ym) -= one_decade) == 1991_y/feb/fri[4]);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decade);
+
+       CPP14_ASSERT(ym + one_decamonth == 2001_y/dec/fri[4]);
+       NOEXCEPT_CONVERSION(ym + one_decamonth);
+       CPP14_ASSERT(one_decamonth + ym  == 2001_y/dec/fri[4]);
+       NOEXCEPT_CONVERSION(one_decamonth + ym);
+       CPP14_ASSERT(ym - one_decamonth == 2000_y/apr/fri[4]);
+       NOEXCEPT_CONVERSION(ym - one_decamonth);
+       CPP14_ASSERT((copy(ym) += one_decamonth) == 2001_y/dec/fri[4]);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decamonth);
+       CPP14_ASSERT((copy(ym) -= one_decamonth) == 2000_y/apr/fri[4]);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decamonth);
+    }
+
+    {
+       constexpr year_month_weekday_last ym = 2001_y/feb/fri[last];
+       CPP14_ASSERT(ym + one_month == 2001_y/mar/fri[last]);
+       NOEXCEPT_ASSERT(ym + one_month);
+       CPP14_ASSERT(one_month + ym == 2001_y/mar/fri[last]);
+       NOEXCEPT_ASSERT(one_month + ym);
+       CPP14_ASSERT(ym - one_month == 2001_y/jan/fri[last]);
+       NOEXCEPT_ASSERT(ym - one_month);
+       CPP14_ASSERT((copy(ym) += one_month) == 2001_y/mar/fri[last]);
+       NOEXCEPT_ASSERT(copy(ym) += one_month);
+       CPP14_ASSERT((copy(ym) -= one_month) == 2001_y/jan/fri[last]);
+       NOEXCEPT_ASSERT(copy(ym) -= one_month);
+      
+       CPP11_ASSERT(ym + one_year == 2002_y/feb/fri[last]);
+       NOEXCEPT_ASSERT(ym + one_year);
+       CPP11_ASSERT(one_year + ym == 2002_y/feb/fri[last]);
+       NOEXCEPT_ASSERT(one_year + ym);
+       CPP11_ASSERT(ym - one_year == 2000_y/feb/fri[last]);
+       NOEXCEPT_ASSERT(ym - one_year);
+       CPP14_ASSERT((copy(ym) += one_year) == 2002_y/feb/fri[last]);
+       NOEXCEPT_ASSERT(copy(ym) += one_year);
+       CPP14_ASSERT((copy(ym) -= one_year) == 2000_y/feb/fri[last]);
+       NOEXCEPT_ASSERT(copy(ym) -= one_year);
+
+       CPP11_ASSERT(ym + one_decade == 2011_y/feb/fri[last]);
+       NOEXCEPT_CONVERSION(ym + one_decade);
+       CPP11_ASSERT(one_decade + ym == 2011_y/feb/fri[last]);
+       NOEXCEPT_CONVERSION(one_decade + ym);
+       CPP11_ASSERT(ym - one_decade == 1991_y/feb/fri[last]);
+       NOEXCEPT_CONVERSION(ym - one_decade);
+       CPP14_ASSERT((copy(ym) += one_decade) == 2011_y/feb/fri[last]);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decade);
+       CPP14_ASSERT((copy(ym) -= one_decade) == 1991_y/feb/fri[last]);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decade);
+
+       CPP14_ASSERT(ym + one_decamonth == 2001_y/dec/fri[last]);
+       NOEXCEPT_CONVERSION(ym + one_decamonth);
+       CPP14_ASSERT(one_decamonth + ym  == 2001_y/dec/fri[last]);
+       NOEXCEPT_CONVERSION(one_decamonth + ym);
+       CPP14_ASSERT(ym - one_decamonth == 2000_y/apr/fri[last]);
+       NOEXCEPT_CONVERSION(ym - one_decamonth);
+       CPP14_ASSERT((copy(ym) += one_decamonth) == 2001_y/dec/fri[last]);
+       NOEXCEPT_CONVERSION(copy(ym) += one_decamonth);
+       CPP14_ASSERT((copy(ym) -= one_decamonth) == 2000_y/apr/fri[last]);
+       NOEXCEPT_CONVERSION(copy(ym) -= one_decamonth);
+    }
+
+}