day increment program

#!/usr/bin/perl
# code to increment day

master(@ARGV);
exit(0);

sub master
{

    my(@days_in_month) = (31,28,31,30,31,30,31,31,31,30,30,31);

    # input format:
    #   15,07/31/2020 0000,778,0,82
    while ($buf = <STDIN>) {
        chomp($buf);

        # split off date and time
        my($date,$time) = split(" ",$buf);

        # split up date
        my($pre,$mon,$day,$year) = split(m%[,/]%,$date);

        # decide if current year is leap year
        my($isleap) = 0;
        {
            # year must be divisble by 4
            last if (($year % 4) != 0);

            # if year is divisible by 100, it is _not_ a leap year unless ...
            if (($year % 100) == 0) {
                # ... it is also divisible by 400
                last if (($year % 400) != 0);
            }

            $isleap = 1;
        }

        # get number of days in february
        if ($isleap) {
            $days_in_month[1] = 29;
        }
        else {
            $days_in_month[1] = 28;
        }

        # change only if time is 0000
        if ($time =~ /^0000,/) {
            # increment day
            $day += 1;

            # advance month if necessary
            if ($day > $days_in_month[$mon - 1]) {
                $day = 1;
                $mon += 1;
            }

            # advance year if necessary
            if ($mon > 12) {
                $mon = 1;
                $year += 1;
            }
        }

        $date = sprintf("%2.2d,%2.2d/%2.2d/%4.4d",$pre,$mon,$day,$year);

        printf("%s %s\n",$date,$time);
    }
}