Improvements to parsing into Utc, converting from other timezones to Utc

[mqudsi]

Thanks for making dealing with dates and times in rust easier and for providing what has become the defacto standard for date-based operations.

I do believe there is room for improvement however when it comes to dealing with DateTime<Utc> instances; unless I'm mistaken and there's actually an easier way, this is what my code currently looks like to parse a timestamp from a string into a DateTime<Utc> instance:

        let created: DateTime<Utc> = DateTime::from_utc(
            DateTime::<FixedOffset>::parse_from_rfc3339("1996-12-19T16:39:57-08:00")
                .unwrap()
                .naive_utc(),
            Utc,
        );

Aside from the fact that it isn't very pretty or terse, I think it's also extremely undiscoverable. In particular, the usage of DateTime::from_utc to convert to a DateTime<Utc> object is unexpected (I understand why it works).

It would be extremely helpful if DateTime<FixedOffset> had a to_utc() helper method or some other means of converting more intuitively between DateTime flavors.

Is there a fundamental reason why there are no std::convert impls to facilitate conversion between these types? Since in the code above there is only the first .unwrap() and all other operations are not expected to ever produce an error, I imagine it would be possible to use std::convert in its current form (vs waiting for std::convert::TryFrom to finally stabilize).

If std::convert::From<chrono::DateTime<chrono::FixedOffset>> were implemented for chrono::DateTime<chrono::Utc> (or std::convert::Into<chrono::DateTime<chrono::Utc>> implemented for chrono::DateTime<chrono::FixedOffset> if one type isn't aware of the other) the code above could be simplified considerably:

        let created: DateTime<Utc> = DateTime::<FixedOffset>::parse_from_rfc3339("1996-12-19T16:39:57-08:00").unwrap().into();

(Although I would still suggest that parse_from_rfcxxxx be implemented for DateTime<Utc> in all cases.)

[dpc]

I came here, looking for help, found this issue so I'll post my experiences.

I was trying to convert date in Local timezone to a FixedOffset timezone.

The timezone/offset conversions are very confusing and very hard to discover. Some of the traits involved don't even make sense to me: like offset vs timezone.

Also, what is Local if not a FixedOffset timezone of system-configured value? Donno. Maybe I'm missing something.

Some simple conversion functions between dates in different timezone, would be greatly appreciated. :)

[quodlibetor]

Hey, sorry about the delay!

I think that the most fundamental question is why there are no From impls for the various timezones. IMO there is no good reason. #67 actually implemented them, but it was from a time when this library was less-maintained. I tried to rebase so that dbrgn would get the original credit, but ran into semantic conflicts.

I believe that when I last gave it a go the design of the Tz traits was such that it was really awkward to implement conversions pleasantly, but I'm not super confident that that's true.

I agree that From should be implemented between all DateTime<T> and parse_from_xxx should be implemented on Utc. A PR implementing those items would be very welcome, otherwise I'll implement it after RustConf.

[mqudsi]

No problem. Thanks for replying!

I started working on this in #271 but I'm pretty sure I'm doing something wrong because the approach I took (.with_timezone()) made it much less painful than I'd expected.

Note that it's not possible to implement the conversion in a single impl<Src, Dst> From<DateTime<Src> for DateTime<Dst> where Src: TimeZone, Dst: TimeZone because of the existing rust impl.

[pitdicker]

Fixed in #271.

Since then you can write:

let created: DateTime<Utc> = DateTime::parse_from_rfc3339("1996-12-19T16:39:57-08:00").into();