let’s say I want the value of “yesterday”, I can d...

# kotlinx-datetime

[jean]

let’s say I want the value of “yesterday”, I can do something like this :

Clock.System.now()
    .minus(1, DateTimeUnit.DAY, TimeZone.of(timeZoneId))

I just realized that now is also relative to my personal timezone, isn’t? I could ask for the now value of japan for example, which is way different that mine. So why doesn’t now take a Timezone parameter? Or did I miss something?

[Dmitry Khalanskiy [JB]]

now returns an Instant, which is timezone-independent and is the same for you and Japan.

[jean]

why does minus et plus cares about the time zone then since it also returns an instant? It’s weird to me that I need to specify the timezone on the operation but not on the starting point

[jean]

Is it not possible to add or substract a duration from an instant without a timezone?

[Desmond van der Meer]

Due to daylight savings etc, 1 day later at the same time may not always be 24 hours.

[Desmond van der Meer]

If you convert the Instant to a LocalDateTime, then you can subtract one day without specifying a timezone

[Dmitry Khalanskiy [JB]]

We don't provide LocalDateTime arithmetic (exactly because it would be possible to use it without a time zone).

[Dmitry Khalanskiy [JB]]

Regarding ±1 day not always being the same as 24 hours, yes, that's the reason. The semantics for ±1 day is "the moment at the next/previous day with the same time of day".

[Desmond van der Meer]

Oops I meant LocalDate 🙂

[Dmitry Khalanskiy [JB]]

Yeah, that's true: Clock.System.now().toLocalDateTime(timeZone).date.minus(1, DateTimeUnit.DAY) also allows knowing what day it was yesterday.

[ilya.gorbunov]

If you're interested only in the date component, Clock.System.todayAt(timeZone) might be of use.

[jean]

I just realized that the time zone parameter is not required in plus and minus which make much more sense to me now hehe. I did not see this was available Instant.minus(value: Long, unit: DateTimeUnit.TimeBased) Thanks for the clarifications 🙂

[Desmond van der Meer]

Note that that function is only available for time-based units (hours, minutes, etc). If you want to subtract a day then you'll still need to specify the timezone.

[jean]

True, I did not see that. Then I’ll just specify it 🤷