Hi! I try to use `kotlinx-datetime` to parse offse...

# kotlinx-datetime

[Vita Sokolova]

Hi! I try to use kotlinx-datetime to parse offset dates like: 2020-02-25T130909+01:00. But I can’t find out how to use
DateTimeFormat<DateTimeComponents> to format LocalDateTime. I looked through docs and found several examples for the opposite conversion, but got stuck with formatting LocalDateTime back to string.

[Dmitry Khalanskiy [JB]]

You can't format just a LocalDateTime using this format because this format includes an offset, and LocalDateTime doesn't include one, so there is nothing to meaningfully format. You can do

offsetDateTimeFormat.format {
  setDateTimeOffset(value, someOffset)
}

but you need to choose the offset to use.

[Vita Sokolova]

Then I definitely do something wrong from very beginning, because the offset is ignored while parsing string to LocalDateTime . What should I do to have an access to the information about offset and do not ignore/lose it?

[Vita Sokolova]

override fun deserialize(decoder: Decoder): LocalDateTime {
        val instant = offsetDateTimeFormat.parse(decoder.decodeString()).toInstantUsingOffset()
        return instant.toLocalDateTime(TimeZone.currentSystemDefault())
    }

will this be better solution?

[CLOVIS]

LocalDateTime is just a datetime with no timezone information at all. It can't ever store anything more than 2020-02-25T13:09:09, because if it did, then by definition it would something else than a LocalDateTime .

If you gave this to an instant, 2020-02-25T13:09:09+01:00 would be stored as 2020-02-25T12:09:09Z. If your goal is to compare two events to know which one happened first, that's the correct data structure. However, it doesn't store in which timezone the event happened.

If you want to store the exact timezone, then you essentially want a Pair<LocalDateTime, FixedOffsetTimeZone> , which I believe is what Java calls a ZonedDateTime but I don't think exists in KotlinX.Datetime (yet?)

[Dmitry Khalanskiy [JB]]

The better solution depends on what you need.
• Instant remembers the moment in time. From its point of view, 2020-02-25T13:09:09+01:00 is exactly the same thing as 2020-02-25T14:09:09+02:00. Use this if you want to know the moment in time, but don't care in which timezone it happens. This is for things like "the moment this e-mail was sent".
• LocalDateTime remembers what the clocks say. It can't distinguish between 2020-02-25T13:09:09+01:00 and 2020-02-25T13:09:09-10:00. Use this if you want to know what the residents of the timezone see on their clocks, but don't care in which timezone it happens. For example, this is ok for local train schedules.
• DateTimeComponents doesn't have specific semantics, it's just a bunch of values grouped together for parsing and formatting. Use it if you need to know both the offset and the moment in time.

[Vita Sokolova]

I’m closer to the 2nd case. I receive date and time of a certain event from the backend and just show it to the user on UI like “You joined 11 Apr 2024 at 12:03”, but our users are located in a few different timezones, so I just want to convert the backend data to LocalDateTime according to my user’s timezone. I believe the code I posted above does it.

[Dmitry Khalanskiy [JB]]

If the offset that's provided by the backend isn't an offset that the users are interested in (but, for example, what the backend itself sees), you can simply use InstantIso8601Serializer and convert to and from Instant, and then to convert the Instant to the LocalDateTime outside of the serializer.

[Dmitry Khalanskiy [JB]]

Typically, you shouldn't convert an Instant to a LocalDateTime until the very last moment when you want to show the datetime to the user. For example, if the user changed the timezone on their device, it doesn't mean that the program should re-request the same data from the server, but the displayed text should update. If you're storing Instant values throughout your program, this is achieved automatically, whereas with LocalDateTime, you have to deal with cache invalidation.

[Vita Sokolova]

ok, will switch to Instant then.

[Vita Sokolova]

thanks for clarifying all this!