A lot of the complexity is to handle localized date formats on systems that support them. Most other implementations of 'date' do not do this.

The easiest way to see this is in US locales, which use 12-hour clocks in GNU 'date' but not other implementations:

  $ date -d '13:00'
  Sat Nov  1 01:00:00 PM PDT 2025
  $ uu_date -d '13:00'
  Sat Nov  1 13:00:00 2025

I added a test case for that recently, since it is a nice usability feature [1].

[1] https://github.com/coreutils/coreutils/commit/1066d442c2c023...