use Date::Calc qw( Nth_Weekday_of_Month_Year );

sub easterndaylight {
    #this function expects
    #YYYY-MM-DD HH:MM:SS with or without leading zeroes.
    #it doesn't have very good error checking.
    my $isodatetime;
    $isodatetime = shift;
    return undef unless $isodatetime;
    my ($date,$time) = split(" ",$isodatetime);
    my ($year,$mon,$day) = split("-",$date);
    my ($hour,$min,$sec) = split(":",$time);
    
    return 0 if ($mon < 4 || $mon > 10);
    return 1 if ($mon > 4 && $mon < 10);
    return undef unless ( ($mon == 4) or ($mon == 10) );

    my ($dststarty,$dststartm,$dststartd);
    my ($dstendy,$dstendm,$dstendd);
    
    if($mon == 4) {

        #first sunday of april:
        ($dststarty,$dststartm,$dststartd) =
            Nth_Weekday_of_Month_Year($year,4,7,1);

        if( $day < $dststartd ) {
            return 0; #not yet
        elsif ( $day > $dststartd ) {
            return 1; #already sprung
        } elsif ( $day == $dststartd ) {
            return 0 if ($hour < 2);
            return 1 if ($hour > 2);
            # there should never be 02:* on a dst
            # switch day, because you should go right
            # from 01:59:59 to 03:00:00...
            # but you never know.
            return 1;
        } else {
            #uhh wtf?
            return undef;
        }
    } elsif ( $mon == 10 ) {

        # fall back on the last sunday of october
        # try fifth sunday first, then try fourth
        ($dstendy,$dstendm,$dstendd) = 
        Nth_Weekday_of_Month_Year($year,10,7,4)
        unless(($dstendy,$dstendm,$dstendd) =
        Nth_Weekday_of_Month_Year($year,10,7,5));

        if( $day < $dstendd ) {
            #not there yet
            return 1;
        } elsif ( $day > $dstendd ) {
            #already fell
            return 0;
        } elsif ( $day == $dstendd ) {
            return 1 if $hour < 1;
            return 0 if $hour > 1;
            # the problem with DST
            # is that on the last sunday
            # in october, it's 01:00:00 through
            # 01:59:59 twice because you fall back,
            # so if you have the time/date from a machine
            # that auto switches for dst or not dst and it
            # doesn't include a timezone identifier,
            # dates in that range are ambiguous.
            # we're going to assume that it hasn't
            # fallen yet.  i hate dst.
            return 1;
        } else {
            #wtf?
            return undef;
        }
            
    } else {
        return undef; #shouldn't make it here.
    }
}
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