I am trying to delete a cookie by reseting it but it wont delete. What am i doing wrong

#!/usr/bin/python
import cgi
import Cookie
import os

def cookie_expiry_date(numdays):
    from datetime import date, timedelta
    new = date.today() + timedelta(days = numdays)
    return new.strftime("%a, %d-%b-%Y 23:59:59 GMT") 




print 'Set-Cookie: UserID=0; expires= '+cookie_expiry_date(-100);

print "Content-type: text/html\n"
print "woooo"