sub abs_day
{
        # this subroutine accepts a month, day, and two-digit year in 
        # mm/dd/yy numerical form, e.g., 3/23/04.
        # it then calculates the "absolute day", 
        # i.e., the number of days
        # since day 0, which is the turn of the 21st century. 
        # so, 1/10/2000 is absolute day 10;       
        # 1/10/2001 is absolute day 376
        # (because year 2000 was a leap year).

        # the routine deals with leap years by
        # multiplying years by 365.25, and then rounding down.

        my $month = shift(@_);
        my $day = shift(@_);
        my $year = shift(@_);

if ($debug)
{
  print "i'm in abs_day and month is $month,", 
        " day is $day, year is $year\n";
}
        my $abs_day;
        undef($abs_day);

        # so each year has 365.25 days
        # years are of the form 00, 01, 02, etc.
        $abs_day = $year * 365.25;
        # add .75 to account for fact that year 00 is a leap year
        $abs_day = $abs_day + .75;

if ($debug)
{
  print "abs_day is $abs_day after adding .75 to yr*365\n";
}


        if ($month == 1) #jan
        {
                $abs_day =  $abs_day + 0;
        }
        if ($month == 2) #feb
        {
                $abs_day = $abs_day + 31;
        }
# starting in march, we add .25 to number of days 
# to account for leap year
        if ($month == 3) #mar
        {
                $abs_day = $abs_day + 59.25;
        }

        if ($month == 4) #apr
        {
                $abs_day = $abs_day + 90.25;
        }
        if ($month == 5) #may
        {
                $abs_day = $abs_day + 120.25;
        }
        if ($month == 6) #jun
        {
                $abs_day = $abs_day + 151.25;
        }
        if ($month == 7) #jul
        {
                $abs_day = $abs_day + 181.25;
        }
        if ($month == 8) #aug
        {
                $abs_day = $abs_day + 212.25;
        }
        if ($month == 9) #sep
        {
        }
        if ($month == 11) #nov
        {
                $abs_day = $abs_day + 304.25;
        }
        if ($month == 12) #dec
        {
                $abs_day = $abs_day + 334.25;
        }

        $abs_day = $abs_day + $day;
if ($debug)
{
  print "now that we have added year, month, and",
        " day, abs_day is $abs_day\n";
}

        $abs_day = sprintf "%d", $abs_day;
if ($debug)
{
  print "now that we have converted from float to integer,",
        " abs_day is $abs_day\n";
}

        return $abs_day;
}
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