Date calculations can be surprisingly tricky.

Pop quiz: What time do you get when you add 2 seconds to 03:59:59 pm on 31/12/2005 in the PST timezone? Let's find out:

  use DateTime;

  my $dt = DateTime->new(
    year   => 2005,
    month  => 12,
    day    => 31,
    hour   => 15,
    minute => 59,
    second => 59,
    time_zone => "America/Los_Angeles",
  );

  $dt->add(seconds => 2);
  print "$dt\n";
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Result is 2005-12-31T16:00:00 since there was a leap second in between.