use DateTime::Format::Strptime;
my $strp = DateTime::Format::Strptime->new(
	pattern => '%Y-%m-%d', time_zone=>'UTC', on_error=>'croak' );
my $dt = $strp->parse_datetime('2017-11-23');
my $is_thanksgiving = $dt->month==11 && $dt->dow==4
	&& $dt->weekday_of_month==4;

##</code><code>##

sub thanksgiving {
	my $year = shift;
	my $dt = DateTime->new(year=>$year,month=>11,day=>1);
	$dt->add(days=>1) while $dt->dow!=4;
	$dt->add(weeks=>3);
	return $dt;
}