The -startdate and -enddate options for the x509 command are display options. You can set specific start and end time using the ca command instead to sign the certificate.
Try something like this:
openssl ca -config /etc/openssl.cnf -policy policy_anything -out clientcert.pem -startdate 120815080000Z -enddate 120815090000Z -cert ca.pem -keyfile cakey.pem -infiles clientcert.csr