format_date needs to be timezone-aware

[homeworkprod]

I'm using Babel's format_date directly in Flask as Flask-Babel is in the process of removing their wrappers (since 1.5 years, but still).

I also tend to use format_date and format_time in tandem instead of just format_datetime as that gives me more flexibility in formatting (e.g. formats medium and short, respectively, to gain 17.03.2021, 14:09, which isn't possible without a custom–and no longer locale-independent–format).

The issue I stumbled upon is that given UTC date/time 2021-03-17T23:00:00, format_datetime will display the day in time zone "Europe/Berlin" (+01:00, at that date) correctly as 2021-03-18 [time here] while format_date will give 2021-03-17.

Since format_date does not get and consider the time zone, the output is wrong (correct me if I made a mistake somewhere). I would actually go as far as saying that this can be considered a (subtle) bug (okay, I'm being dramatic), or at least a serious limitation.

The reason I'm mentioning Flask-Babel is that it's format_date wrapper indeed does rebase according to the time zone (at least for the time being [no pun intended]). Due to the aforementioned removal of its formatters, Flask-Babel users will probably run into this trap (and might not even learn about it).

My suggestion is to add an optional tzinfo argument to Babel's format_date, so that one can create their own wrapper:

def format_date(*args) -> str:
    return babel.dates.format_date(
        *args, locale=get_locale(), tzinfo=get_timezone()
    )

I also understand the perspective that a date formatter only has to know about dates, not times. However, I think I made a case or two here while an (optional) broader view might make sense.

Thanks for reading! Please let me know what you think. :)