date: --debug option not yet implemented

The date --debug option is not yet implemented. Here's an example of the output:

 TZ=America/Belize date --debug -d 'TZ="Asia/Tokyo" Sun, 90-12-11 + 3 days - 90 minutes' +'%a %b %e %T %z %Y'
date: parsed day part: Sun (day ordinal=0 number=0)
date: parsed date part: (Y-M-D) 0090-12-11
date: parsed relative part: +3 day(s)
date: parsed relative part: +3 day(s) -90 minutes
date: input timezone: TZ="Asia/Tokyo" in date string
date: warning: adjusting year value 90 to 1990
date: warning: using midnight as starting time: 00:00:00
date: warning: day (Sun) ignored when explicit dates are given
date: starting date/time: '(Y-M-D) 1990-12-11 00:00:00'
date: warning: when adding relative days, it is recommended to specify noon
date: after date adjustment (+0 years, +0 months, +3 days),
date:     new date/time = '(Y-M-D) 1990-12-14 00:00:00'
date: '(Y-M-D) 1990-12-14 00:00:00' = 661100400 epoch-seconds
date: after time adjustment (+0 hours, -90 minutes, +0 seconds, +0 ns),
date:     new time = 661095000 epoch-seconds
date: timezone: TZ="Asia/Tokyo" environment value
date: final: 661095000.000000000 (epoch-seconds)
date: final: (Y-M-D) 1990-12-13 13:30:00 (UTC)
date: final: (Y-M-D) 1990-12-13 22:30:00 (UTC+09)
date: output format: ‘%a %b %e %T %z %Y’
Thu Dec 13 07:30:00 -0600 1990

Most of these messages would need to be implemented in the parse_datetime library, where the parsing happens.

Start in the parse_datetime library, where the issue says date parsing happens, and compare its current parsing stages with the example debug output. Identify which listed messages are not yet emitted and determine how the date command should expose them. Done means date --debug produces the requested diagnostic output for the example invocation.