openssl provides the -no_check_time option to "ignore certificate validity time".
Example:
$ openssl verify -CAfile cert1.pem -CApath ./ cert2.pem
...
error 10 at 1 depth lookup: certificate has expired
...
error 10 at 0 depth lookup: certificate has expired
error cert2.pem: verification failed
$ openssl verify -no_check_time -CAfile cert1.pem -CApath ./ cert2.pem
cert2.pem: OK
It would be useful to have a "no check time" option in xmlsec too.