This is actually several separate C++11/C++14/C++17 libraries:
* "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.
* "tz.h" / "tz.cpp" 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.
* "iso_week.h" is a header-only library built on top of the "date.h" library
  which implements the ISO week date calendar.
* "julian.h" is a header-only library built on top of the "date.h" library which
  implements a proleptic Julian calendar which is fully interoperable with
  everything above.
* "islamic.h" is a header-only library built on top of the "date.h" library
  which implements a proleptic Islamic calendar which is fully interoperable
  with everything above.