Binarus wrote: >As the documentation tells us, DateTime always chooses the later time >when calculating with ambiguous times,
This logic is actually in DateTime::TimeZone, where DateTime invokes it via the ->offset_for_local_datetime method. The internal logic is able to walk the sequence of observances, and when it finds a matching observance it looks ahead to the next one to check specially for ambiguity. It returns the later span if there's an ambiguity due specifically to a change from DST to non-DST; it doesn't have guaranteed behaviour in any other ambiguity scenario. DT:TZ doesn't document this feature, and doesn't offer any other form of ambiguity detection in its API (though of course it would be possible to add some). > and if you subtract an hour from >the later (ambiguous) time, you'll get the same time, but the earlier >one (provided the clock is turned back by an hour when switching back). You can't rely on offset changes, even specifically ones for DST, being of an hour. Australia/Lord_Howe (Lord Howe Island) does a regular half-hour DST jump. -zefram
