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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. See http://howardhinnant.github.io/date/date.html for more details.
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. See http://howardhinnant.github.io/date/tz.html for more details.
3.`"iso_week.h"` is a header-only library built on top of the `"date.h"` library which implements the ISO week date calendar. See http://howardhinnant.github.io/date/iso_week.html for more details.
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. See http://howardhinnant.github.io/date/julian.html for more details.
There has been a recent change in the library design. If you are trying to migrate from the previous design, rename `day_point` to `sys_days` everywhere, and that ought to bring the number of errors down to a small roar.
