Sources for datetime are adapted from Howard Hinnant's date library (https://github.com/HowardHinnant/date).

Sources are taken from changeset 2e19c006e2218447ee31f864191859517603f59f of the above project.

The following changes are made:

• fix internal inclusion paths (from "date/xxx.h" to simply "xxx.h")
• enclose the date namespace inside the arrow_vendored namespace
• include a custom "visibility.h" header from "tz.cpp" for proper DLL exports on Windows
• disable curl-based database downloading in "tz.h"