A long while back, after finding these:

I put together the following subs. Please note that I add or subtract 3 in the conversion routines. This allows a modulo function to return the day of the week with 0 equaling Sunday. As in:

$DoW = $gdate % 7;
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I humbly submit the following for review...

#===================================================================
#   d2g() Convert year, month, and day to date factor G.

sub d2g {
    my($y, $m, $d) = @_;
    my $g;
    $m = ($m + 9) % 12;
    $y = $y - int($m/10);
    $g = 365*$y + int($y/4) - int($y/100) + int($y/400) + int(($m*306 
++ 5)/10) + ($d - 1);
    #-------
    $g += 3;
    return $g;
    }

#====================================================================
#   g2d() Convert date factor G to year, month, and day.

sub g2d {
    my($g) = @_;
    my($y, $m, $d, $M, $D);
    #-----------------------------------
    $g -= 3;
    #-------
    $y = int((10000*$g + 14780)/3652425);
    $D = $g - (365*$y + int($y/4) - int($y/100) + int($y/400));

    if  ( $D < 0 ) {
        $y = $y - 1;
        $D = $g - (365*$y + int($y/4) - int($y/100) + int($y/400));
        }

    $M = int((100*$D + 52)/3060);
    $m = ($M + 2)%12 + 1;
    $y = $y + int(($M + 2)/12);
    $d = $D - int(($M*306 + 5)/10) + 1;
    return ($y, $m, $d);
    }

#====================================================================
#   ValidYMD() Returns true if given year, month, and day is valid.

sub ValidYMD {
    my($y, $m, $d) = @_;
    my($Y, $M, $D) = g2d( d2g($y, $m, $d) );
    return $y == $Y && $m == $M && $d == $D;
    }
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