Suggestions for TzDateTime Enhancements

[hkalexling]

In my application I need to work with timezones, and I found TzDateTime in the standard library a bit lacking.

1. from_date equivalent with timezone information

It would be great to have a function similar to datetime::from_date but with timezone information. Currently I need to write something like this:

fn TzDateTime from_date_tz(int year, Month month = JANUARY, int day = 1, int hour = 0, int min = 0, int sec = 0, int us = 0, int gmt_offset = 0)
{
    DateTime dt = from_date(year, month, day, hour, min, sec, us);
    dt.time -= (Time)gmt_offset * SEC;
    return { dt, gmt_offset };
}

2. timezone-aware add_* methods

It would be useful to have TzDateTime counterparts for the add_* methods. While we can already call these methods on TzDateTime (since TzDateTime has inlined DateTime), we must manually adjust the returned values to maintain timezone consistency. For example:

	TzDateTime now = datetime::now().to_local();

	io::printn(now); // 2024/SEPTEMBER/23 13:50:51 +7

	// This is enough for add_days etc., where we directly work with `time`
	io::printn(now.add_days(10).to_local()); // 2024/OCTOBER/3 13:50:51 +7

	// If we do this for add_months and add_years, it produces incorrect result
	io::printn(now.add_months(1).to_local()); // 2024/OCTOBER/23 20:50:51 +7

	// Instead we have to do
	TzDateTime dt = { now.add_months(1), now.gmt_offset };
	io::printn(dt); // 2024/OCTOBER/23 13:50:51 +7

3. to_gmt_offset method

A to_gmt_offset method for both DateTime and TzDateTime would be useful. This would function similarly to to_local, but allow specifying a custom GMT offset.

If these ideas align with the direction of c3 I'd be happy to submit a PR.

[lerno]

Yes, those sounds like good additions.