This is actually two separate C++11/C++14 libraries:

1.  `"date.h"` is a header-only library which builds upon `<chrono>`.  It adds some new `duration` types, and new `time_point` types.  It also adds "field" types such as `year_month_day` which is a struct `{year, month, day}`.  And it provides convenient means to convert between the "field" types and the `time_point` types.  See http://howardhinnant.github.io/date_v2.html for more details.

2. The other files are a timezone library built on top of the `"date.h"` library.  This timezone library is a complete parser of the IANA timezone database.  It provides for an easy way to access all of the data in this database, using the types from `"date.h"` and `<chrono>`.  The IANA database also includes data on leap seconds, and this library provides utilities to compute with that information as well.  See http://howardhinnant.github.io/tz.html for more details.