How to assign to a new variable the current time using time::PrimitiveDateTime type?

[frederikhors]

I'm using time::PrimitiveDateTime and I would like to create a new var with the current time but I cannot find how. Isn't there something like now()?

What about Instant::now()?

Example:

pub struct Player {
  updated_at: Option<PrimitiveDateTime>
}

impl Player {
    pub fn set_updated_at(&mut self) {
        let now = Instant::now();

        self.updated_at = Some(time::PrimitiveDateTime::from(now));
    }

But obviously this doesn't work:

mismatched types
expected struct `time::PrimitiveDateTime`, found struct `time::Instant` rustc E0308

[jhpratt]

To get the current time, you have to use OffsetDateTime — specifically OffsetDateTime::now_utc or OffsetDateTime::now_local. This is because Instant is opaque and PrimitiveDateTime has no time zone or UTC offset — something necessary to indicate a specific point in time.

[Kinrany]

What is the canonical place to read the reasons time shouldn't be stored in UTC, or that PrimitiveDateTime shouldn't be used for that purpose despite being mathematically equivalent?

[jhpratt]

You are free to store it as if it is UTC, but that doesn't mean time is aware that's what you're doing. Having no offset and having an offset of UTC are not equivalent, just as None that is of type Option<u32> is not equivalent to zero.

[Kinrany]

I guess the reason people often try to use PrimitiveDateTime is that it's very common to store timestamps in UTC, and in these cases storing OffsetDateTime is a footgun: no way to enforce at compile time that the offset is zero other than by wrapping it in a newtype. PrimitiveDateTime is the only obvious alternative that has the correct shape.

This made me realize an important difference between time and chrono: time zones are dynamic in time and static in chrono. We probably need both.

Edit: chrono does allow dynamic time zones as one of the time zone types, hmm.

[jhpratt]

I have mulled adding a UtcDateTime, for what it's worth. A recent internal change would make it very easy to implement the guts, and I'd just need to write the façade.

More generally, having support for an arbitrary static offset is something I hope to achieve eventually. Unfortunately at this point, implementing it would require a few nightly features to get it looking (and working) the way I want.