Support changing OffsetDateTime's offset without changing the actual instant in time.

[vincentdephily]

I'm parsing a unix timestamp and formatting it for the current timezone. This involves calling replace_offset() then checked_add() :

let fmt = format_description!("[year]-[month]-[day] [hour]:[minute]:[second] [offset_hour sign:mandatory]:[offset_minute]");
let offset_secs = Duration::new(offset.whole_seconds().try_into().unwrap(), 0);
OffsetDateTime::from_unix_timestamp(ts).unwrap()
                                       .replace_offset(offset)
                                       .checked_add(offset_secs)
                                       .unwrap()
                                       .format(&fmt)
                                       .unwrap()

It would be nice and less error-prone to have a set_offset() function that covers both operations.

It would be even nicer to have a set_timezone() function as this code is not DST-aware (and my program is multithreaded at this stage), but I understand that this is a much larger ask (#380).

[jhpratt]

Is this not OffsetDateTime::to_offset?

[vincentdephily]

D'oh ! Yes, to_offset, I looked for it hard but missed it. Sorry for the noise, closing.