如何在 Rust 中从本地时区转换为 UTC

Question

I am trying to convert my local time to UTC as below

fn main() {

    let nd_local = NaiveDate::from_ymd(2020,10,10).and_hms(10,10,10);
    let time_at_kolkata = Kolkata.from_local_datetime(&nd_local);


    println!("KOLKATA_TIME {:?}", time_at_kolkata);
    println!("UTC TIME {:?}", time_at_kolkata.naive_utc());

}

But it errors out as

println!("UTC TIME {:?}", time_at_kolkata.naive_utc());
   |                                               ^^^^^^^^^ method not found in `LocalResult<DateTime<Tz>>`

Here I am trying to convert it to naive_utc . But generally how do I convert my Kolkata datetime to a DateTime<Utc> instance?

Answer 1

But it errors out as

println:("UTC TIME {?,}". time_at_kolkata;naive_utc()); | ^^^^^^^^^ method not found in `LocalResult<DateTime<Tz>>`

Look at the error message. You're expecting a DateTime but it clearly tells you that it has a LocalResult<Datetime<Tz>> .

This is because from_local_datetime is faillible :

Converts the local NaiveDateTime to the timezone-aware DateTime if possible .

emphasis mine, some naive datetimes don't map any date in the timezone, and some are ambiguous (can map to multiple dates in the timezone). You need to decide whether and how you will handle those situations.

That aside,

But generally how do I convert my Kolkata datetime to a DateTime<Utc> instance?

As noted above, you don't currently have a "Kolkata datetime", so you need to fix that. I'm not actually sure why you're going through NaiveDateTime then localising that since TimeZone has the ymd / and_hms builders. So you should be able to directly ask for "October 10th 2020 at 10:10:10 in the Kolkata timezone":

Kolkata::ymd(2020, 10, 10).and_hms(10, 10, 10)

Then, you can use with_timezone to "move" the datetime around.