Ensure better ISO 8601 compliance #330
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ISO 8601 defines the period of one month as a fixed number of calendar days, specified by the calendar. Before this change, we treated a month as a separate time scale unit during `DatePeriod` arithmetics, namely, we resolved the `LocalDateTime` on which we did arithmetics internally after adding the months but before adding the days. This change makes it so that adding months and days is done as a single change, only consulting the calendar, and the time zone is only used once all calendar-based operations are performed. The code that calculates the `DateTimePeriod` between two `Instant` values is changed symmetrically.
ilya-g
reviewed
Dec 14, 2023
| throw DateTimeArithmeticException("The number of months between $this and $other does not fit in an Int") | ||
| } | ||
| thisDate = thisDate.plus(days, DateTimeUnit.DAY) // won't throw: thisLdt + days <= otherLdt | ||
| val thisInstant = LocalDateTime(thisDate, thisLdt.time).toInstant(timeZone) |
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Does this toInstant use the same resolve mode as ZDT.plus(days) that was used before?
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Not exactly.
Imagine a situation:
- UTC offset before is
+1:00; - An overlap happens;
- UTC offset after is
+0:00.
If we subtract, say, a day and land into this overlap, then, as is currently implemented, the +0:00 offset will be used. In this PR, the UTC offset +1:00 will be used instead.
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I believe it would be better to preserve resolve behavior in these operations, so that
Instant + DatePeriod(days = 1) == Instant.plus(1, DAY)
| @@ -11,6 +11,7 @@ import kotlinx.datetime.internal.* | |||
| import kotlinx.datetime.serializers.InstantIso8601Serializer | |||
| import kotlinx.serialization.Serializable | |||
| import java.time.DateTimeException | |||
| import java.time.ZonedDateTime | |||
dkhalanskyjb
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dkhalanskyjb
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ISO 8601 defines the period of one month as a fixed number of calendar days, specified by the calendar.
Before this change, we treated a month as a separate time scale unit during
DatePeriodarithmetics, namely, we resolved theLocalDateTimeon which we did arithmetics internally after adding the months but before adding the days.This change makes it so that adding months and days is done as a single change, only consulting the calendar, and the time zone is only used once all calendar-based operations are performed.
The code that calculates the
DateTimePeriodbetween twoInstantvalues is changed symmetrically.