Inconsistent timezone offset handling with GNU date

[yuankunzhang]

GNU date accepts a timezone code plus an offset between -24 and +24 (e.g. m+24, y-24).

We are using chrono::FixedOffset to represent offsets, which accepts offsets only in the range -24 to +24. Note that the literal m+12 will overflow because the letter m already denotes a +12 offset.

We should re-design the underlining data models to achieve the GNU date behavior.

[Maximkaaa]

Also, even though chrono supports setting offsets up to 24 hours, parsing for more than 12:59 doesn't work. For example, parsing:

11:34:56+13:00

produces error in parse_datetime, but handled correctly in GNU.