# $d, $m and $y have come from the web form. They will
# be (for example) $d = 09, $m = 10, $y = 2002.
# First we need to convert them to "timelocal-friendly"
# values.
$m--;
$y -= 1900;

# Then use eval to check date is valid
eval "timelocal 0, 0, 0, $d, $m, $y";

# Warn on errors
die "Invalid date\n" if $@;