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use DateTime::Format::Strptime;
my $strp = DateTime::Format::Strptime->new(
...
my $dt = $strp->parse_datetime('2017-11-23');
my $is_thanksgiving = $dt->month==11 && $dt->dow==4
    && $dt->weekday_of_month==4;

sub thanksgiving {
    my $year = shift;
...
    $dt->add(weeks=>3);
    return $dt;
}