Truncate ignores DST flag. Bug or intended behavior?

[bbkr]

use DateTime;
my $dt = DateTime->new( "year" => 2022, "month" => 10, "day" => 30, "hour" => 1, "time_zone" => "Europe/Dublin" );
say $dt->is_dst;
$dt->add( "hours" => 1 );
say $dt->is_dst;
$dt->truncate( "to" => "hour" );
say $dt->is_dst;

Output:

0
1
0

So truncate rewinded time back to hour before DST gap.

• From relative time perspective it looks incorrect - there is nothing to truncate in 2022-10-30T01:00:00.
• From absolute time perspective it looks correct - there are extra 3600 seconds that could be chopped to reach 2022-10-30T01:00:00.

If this is correct behavior i think it should be mentioned in truncate and set methods.

[bbkr]

This may be related to some weird bug - looks like DST flag for Dublin is... backwards: #136

[bbkr]

Update: The direct reason of described behavior is edge-case time zone with negative DST. That is OK, however this low level logic is based on some assumption about DST change direction that makes user experience very, very weird:

• time zone X => +1 hour on DST overlap => truncate to hour = 3600 seconds difference
• time zone Y => +1 hour on DST overlap => truncate to hour = 0 seconds difference

Those are the same operations! So if current behavior is considered correct at least documentation should explain how DST impacts truncate. For example:

---

Truncate will always prefer Standard Time over Daylight Saving Time.

For example 2022-10-30 02:42:00 Europe/Warsaw summer time + 1 hour is 2022-10-30 02:42:00 Europe/Warsaw winter time. And 2022-10-30 02:42:00 Europe/Warsaw winter time truncated to hour is 2022-10-30 02:00:00 Europe/Warsaw still in winter time.

However rare cases like Europe/Dublin time zone use negative Daylight Saving Time where winter is considered the DST period as explained here. In those cases truncate will go back to Standard Time. For example 2022-10-30 01:42:00 Europe/Dublin summer time + 1 hour is 2022-10-30 01:42:00 Europe/Dublin winter time. But 2022-10-30 01:42:00 Europe/Dublin winter time truncated to hour is 2022-10-30T01:00:00 Europe/Dublin summer time. 1h 42min were truncated because 01:00:00 in summer Standard Time is preferred over 01:00:00 in winter Daylight Saving Time.

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I know I'm being pushy about explicit docs here, but I've spent few hours on "WTF debugging" when my script got into infinite loop - it was not expecting truncate to subtract more than 1 hour. And I had extensive DST skips tests in other time zones so I thought everything was covered. Nobody expects Spanish Inquisition and negative DST logic :)

[autarch]

I would welcome a PR with a doc patch!