DateTime: Enhancements

[sharkdp]

A few enhancements that I thought about while reviewing #287. Let's discuss them here and potentially implement later:

• "local" should represent the local timezone, not the local UTC offset
• parse_datetime should work without a UTC offset (and use the local timezone to get the offset)
• parse_datetime should work for more relaxed input formats. For example, I often want to leave out seconds. I also find the T separator between the date and the time rather unreadable (yes, I know, it's the official standard). https://crates.io/crates/chrono_parser might be useful
• There should probably be a way to enter dates in another timezone (we can already specify UTC offsets, but not timezones)
• Documentation
• We currently print the UTC offset when showing date times. Personally, I'd rather see the current clock time (e.g. "Central European Time" or "Central European Summer Time", depending on the time of the year) instead.
• Make TZ environment variable work (it used to, probably broken with the timezone vs offset change)
• Comparison operators like ==, !=, >, etc. should probably work on date times
• It would be cool to have a shorthand syntax to enter dates. Something like @2024-02-08 or @2024-02-08@ or d"2024-02-08". The latter might allow us to add support for entering other things in the future. The @ syntax might be problematic, as we already use @ for decorators.
• Displaying of dates should potentially be configurable, or automatically adjusted using localization features. In the US, we might want to show a 12-hour based AM/PM time, in Europe, we might want to show a 24-hour based time. Chrono even has a locale feature (even though it's currently considered unstable), which would allow showing names of months/weekdays in the proper language. We do not have l10n features anywhere else in Numbat, so I'm also okay with skipping that part.
• Potentially better error messages for things like now() - 1
• Add accessors for datetime properties, like year, month, day, hour, minute, second, weekday, ordinal. They could be used as conversion functions: now() -> weekday
• Untangle DateTime and Time/second by introducing a @lang or @register decorator to associate the Time dimension and the second unit with their special role.
• Resolve other TODOs from original implementation

A few useful functions would be:

• parse_date (assuming 00:00:00 in the local timezone)
• Maybe today, which would be the same thing as now(), but with a time of 00:00:00?
• A two-argument version of parse_datetime that also takes a format string?

[RossSmyth]

Would probably good to take some inspiration from burntshushi's new lib
https://github.com/BurntSushi/jiff

[sharkdp]

Would probably good to take some inspiration from burntshushi's new lib

Yes! I thought the same thing when I read about it. Would be interesting to port from chrono to jiff as a small experiment. jiff might be beneficial for us due to its TZ database handling strategy, which would solve #344 after all.

[BurntSushi]

Would be interesting to port from chrono to jiff as a small experiment.

I'm doing some of these myself and adding APIs to Jiff as a result of those experiments. So if you do this, please reach out. I'd love to hear feedback. I'm keen to iterate for the next ~year and get Jiff to 1.0.

[BurntSushi]

Looking at your list, I believe Jiff should support doing everything there, except for automatic locale support. For locale, I'm hoping an integration with icu will make that work, although I haven't done any exploration there.

[sharkdp]

@BurntSushi I ported Numbat from chrono to jiff here: #511

I wrote some comments, which might be helpful for you to get some insights into the chrono-to-jiff journey by someone who's not yet familar with jiff.

[sharkdp]

The jiff PR has now been merged, which checks off two items from our list:

• We currently print the UTC offset when showing date times. Personally, I'd rather see the current clock time (e.g. "Central European Time" or "Central European Summer Time", depending on the time of the year) instead.
• Make TZ environment variable work