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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.
2.`"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.
4.`"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.
5.`"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.
* Status: The proposal was approved by LEWG and forwarded to LWG with the recommendation of targeting C++20 at the Albuquerque, NM meeting in Nov. 2017.