There are edge cases where rounding a date to a certain interval using a time
zone with DST shifts can currently cause the rounded date to be bigger than the
original date. This happens when rounding a date closely after a DST start and
the rounded date falls into the DST gap.
Here is an example for CET time zone, where local time is set forward by one
hour at 2016-03-27T02:00:00+01:00 to 2016-03-27T03:00:00.000+02:00:
The date 2016-03-27T03:01:00.000+02:00 (1459040460000) which is just after the
DST change is first converted to local time (1459047660000). If we then apply
interval rounding for a 14m interval in local time, this takes us to
1459047240000, which unfortunately falls into the DST gap. When converting
this back to UTC, joda provides options to throw exceptions on illegal dates
like this, or correct this by adjusting the date to the new time zone offset.
We currently do the later, but this leads to converting this illegal date back
to 2016-03-27T03:54:00.000+02:00 (1459043640000), giving us a date that is
larger than the original date we wanted to round.
This change fixes this by using the "strict" option of 'convertLocalToUTC()'
to detect rounded dates that fall into the DST gap. If this happens, we can use
the time of the DST change instead as the interval start.
Even before this change, intervals around DST shifts like this can be shorter
than the desired interval. This, for example, happens when the requested
interval width doesn't completely fit into the remaining time span when the DST
shift happens. For example, using a 14m interval in UTC+1 (CET before DST
starts) leads to the following valid rounding values around the time where DST
happens:
2016-03-27T01:30:00+01:00
2016-03-27T01:44:00+01:00
2016-03-27T01:58:00+01:00
2016-03-27T02:12:00+01:00
2016-03-27T02:26:00+01:00
...
while the rounding values in UTC+2 (CET after DST start) are placed like this
around the same time:
2016-03-27T02:40:00+02:00
2016-03-27T02:54:00+02:00
2016-03-27T03:08:00+02:00
2016-03-27T03:22:00+02:00
...
From this we can see then when we switch from UTC+1 to UTC+2 at 02:00 the last
rounding value in UTC+1 is at 01:58 and the first valid one in UTC+2 is at
03:08, so even if we decide to put all the dates in between into one rounding
interval, it will only cover 10 minutes. With this change we choose to use the
moment of DST shift as an aditional interval separator, leaving us with a 2min
interval from [01:58,02:00) before the shift and an 8min interval from
[03:00,03:08) after the shift.
This change also adds tests for the above example and adds randomization to the
existing TimeIntervalRounding tests.
5abe1f7bb2