Field Default Values UI: Time

https://git.drupalcode.org/project/xdate/-/blob/c4c9995862a16869c117ce21...

Problem/Motivation

The default value for start and end times is an example of a non-HTML5 date input for the element. It needs to be able to support absolute dates. It also supports relative dates. It could also support tokens? It can also support different accuracy: you can default the date part only of a field that requires the date and time.

Proposed resolution

Create a form field, probably with some helpers, to set the default date.

Remaining tasks

• Document the options.
  https://git.drupalcode.org/project/xdate/-/blob/1.x/tests/src/Kernel/Xda...
  https://git.drupalcode.org/project/xdate/-/blob/c4c9995862a16869c117ce21...
• Create the field.