diff --git a/include/date/date.h b/include/date/date.h
index 4ad85cc..6960e8c 100644
--- a/include/date/date.h
+++ b/include/date/date.h
@@ -4206,8 +4206,8 @@ template <class CharT, class Traits, class Duration>
 inline
 typename std::enable_if
 <
-    std::ratio_less<typename Duration::period, days::period>::value
-    , std::basic_ostream<CharT, Traits>&
+    !std::is_convertible<Duration, days>::value,
+    std::basic_ostream<CharT, Traits>&
 >::type
 operator<<(std::basic_ostream<CharT, Traits>& os, const sys_time<Duration>& tp)
 {