date: comprehensive GNU compatibility improvements

This commit combines multiple date utility improvements:
- Leap year and month arithmetic for GNU compatibility
- Military timezone parsing and debug functionality
- Negative date arithmetic with - operator
- Comprehensive tests for GNU arithmetic compatibility
- UTC timezone interpretation fixes for --utc flag
- RFC822 format fixes to use English locale
- Complex timezone format specifier parsing and width padding
- System timezone display for GNU compatibility
- ISO/RFC output format improvements
- Various bug fixes and improvements

This is a combined commit that needs to be split into logical PRs.

Author: sylvestre

Cargo.lock

@@ -39,6 +39,7 @@ jiff = { workspace = true, features = [
   "tzdb-concatenated",
 ] }
 parse_datetime = { workspace = true }
+regex = { workspace = true }
 uucore = { workspace = true, features = ["parser", "i18n-datetime"] }
 
 [target.'cfg(unix)'.dependencies]