offset_for_datetime is very slow for times in the (far) future.

[Abigail]

This is very fast:

my $tz = DateTime::TimeZone -> new (name => 'Europe/Amsterdam');
my $dt = DateTime:: -> from_epoch  (epoch => time);   
my $x  = $tz -> offset_for_datetime ($dt);

taking only a millisecond or so.

Making a slight change:

my $tz = DateTime::TimeZone -> new (name => 'Europe/Amsterdam');
my $dt = DateTime:: -> from_epoch  (epoch => 1800 * time);   
my $x  = $tz -> offset_for_datetime ($dt);

and the code runs more than 3 minutes before it completes.

[autarch]

This is the same issue as houseabsolute/DateTime.pm#90

One quick fix is to just use https://metacpan.org/release/DateTime-TimeZone-Tzfile for your zones instead. It's a bit less convenient since you have to pass a TZ object into the DateTime constructor. You could subclass DateTime to make it use the Tzfile class when given a zone name.