Question

I’m working on a world clock web application, and I’d like to add a function to note how and when each location will next undergo a DST change.

I’m using the DateTime and DateTime::Timezone CPAN modules, which seem to provide every conceivable function apart from this one! DateTime::Timezone is an implementation of the tz/Olson database.

So far, I’m comparing local offset to UTC, both now and 24 hours ahead, which allows me to flag what the effect is but not exactly when it occurs.

My proof of concept calculates this every minute. I’m thinking the next step is either:

• create a lookup table using a similar method for the next 365 days to identify affected days, then look at each hour within those days

• somehow parse the DST rules in the DateTime::Timezone module.

Any ideas? I should note that my knowledge of Perl is pretty basic.

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Edit:

Looks like another option might be to port to PHP and use DateTimeZone::getTransitions. This did return an array of all transitions, but PHP 5.3.0 just added optional timestamp_begin and timestamp_end paramaters to limit to a range.

Answer 1

The undocumented attribute _rules of an DateTime::TimeZone object gives you an array of DateTime::TimeZone::OlsonDB::Rule objects. These encode the start and end of observances.

my $o = DateTime::TimeZone->new(name => 'Europe/Vienna')->_rules->[1];
say sprintf "%s summertime ends at %s on %s in %s each year", $o->name, $o->at, $o->on, $o->month;

I advise you to file a bug to request that this API is made public and documented, so that your code can rely on it and it is going to be supported by the DT maintainers properly.