This is basic guesswork, but I have to ask: did you verify that the right thing is getting into your cgi program?

$date = param('date');
print "got date $date in cgi";
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I guess the other basic thing to check would be to use your database's command line tool to make sure that it recognized the date format of what your feeding it as a date.
Your database might have very specific rules on how to format dates.