Rust

It’s not built-in support.

Right. That's exactly what the code snippet says:

    // The serialized datetime has no time zone information,
    // so unless there is some out-of-band information saying
    // what its time zone is, we're forced to use a fixed offset:

So I feel like the point you're making here is already covered by the example comparison I wrote. It's not built-in, so you have to invent your own interchange format. And since your serialized format doesn't include offset information at the time the instant was created, it's impossible to do offset conflict resolution. For example, let's say you record one year from today in Ukraine:

use jiff::{ToSpan, Unit, Zoned};

fn main() -> anyhow::Result<()> {
    let now = Zoned::now().round(Unit::Minute)?.intz("Europe/Kyiv")?;
    let next_year = now.checked_add(1.year())?;
    println!("{next_year}");
    Ok(())
}

And the output:

$ cargo -q r
2025-07-22T17:23:00+03:00[Europe/Kyiv]

And maybe you store this datetime somewhere.

At this point, it's looking like Ukraine is going to abolish DST for next year. So what happens to that datetime above? It no longer has the right offset. So now you need to choose whether to reject it altogether (the default), respect the offset (even if the civil time changes) or respect the civil time (even if the instant changes).

Here's an example of when this happened with Brazil abolishing DST: https://docs.rs/jiff/latest/jiff/fmt/temporal/struct.DateTimeParser.html#example-3