my @monthDays= qw( 31 28 31 30 31 30 31 31 30 31 30 31 );
sub MonthDays {
    my $month= shift(@_);
    my $year= @_ ? shift(@_) : 1900+(localtime())[5];
    if(  $year <= 1752  ) {
        # Note:  Although September 1752 only had 19 days,
        # they were numbered 1,2,14..30!
        return 19   if  1752 == $year  &&  9 == $month;
        return 29   if  2 == $month  &&  0 == $year % 4;
    } else {
        return 29   if  2 == $month  and
          0 == $year%4  &&  0 == $year%100  ||  0 == $year%400;
    }
    return $monthDays[$month-1];
}
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This covers Julian and Gregorian dates and uses Julian month lengths even prior to the Julian calendar.