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.