On 11.07.2017 01:09, Zefram wrote: > Binarus wrote: >> Using DateTime, is it possible to tell in advance if a certain date-time >> which is given in a certain locale will be ambiguous due to switching >>from DST to standard time? > > That is tricky. I don't think our APIs provide any way to do it. > Thinking about the facilities available a bit lower down, the way I'd > probably approach it is to look at the list of all the offsets ever used > in that timezone (not available through any API, but extractable from > the tzfile). I'd compute for each the UT time that would be represented > by the specified local time with that offset, and then use the regular > API to convert that UT time to the local time in the specified timezone > (or equivalently just to look up the zone's offset for that UT time). > Look at which local times come out matching the specified local > time (which offsets match the candidate offset that we were trying). > If there's more than one match, that's an ambiguous local time. If there > are no matches, it's a non-existent local time.
At first, thank you very much for your answer! I see. I always thought that the tzfile only contained the offsets for standard time (and not DST). Obviously, I have been wrong (I never had a look into a tzfile yet because I hope that I could solve my problem without reading it out directly). > Something close to this can actually be done in the C time API. You > don't get to ask what all the zone's offsets are, but in the local->UT > conversion you can specify whether DST is in effect. Giving both states > of that flag gives you two UT times, which you can then convert back > to local to check whether they come out with the same DST flag state. > This will work for regular DST changes, but not for offset changes that > are unrelated to DST. This is very interesting. Actually, I am doing more things in C than in Perl (but on other architectures), but since I thought that DateTime was mirroring all capabilities libc has in this respect, I did not look into the C time API yet. I have no dislike against writing a part of the application in C (but then will have to learn how to call C from Perl). Again, thanks for all advice, Binarus
