How do I determine the expiration date of a p12 certificate?

First you will need to translate the pkcs12 certificate into a PEM certificate.
The PEM certificate is only needed temporarily and can later be removed.

$ openssl pkcs12 -in certificate.p12 -out tempcrt.pem
Enter Import Password:
MAC verified OK
Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:

Now, we use the tempcrt.pem that we generated to determine the expiration date. The first method, which only displays the expiration date can be retrieved like this:

$ openssl x509 -in tempcrt.pem -noout -enddate
notAfter=Jan  3 23:19:24 2009 GMT

The second method which includes a lot more detail about the certificate below, I’ve only included the details relevant to the creation and expiration dates.

$ openssl x509 -in tempcrt.pem -noout -text
        [ ... snip ... ]
        Validity
            Not Before: Jan  3 23:19:24 2008 GMT
            Not After : Jan  3 23:19:24 2009 GMT
        [ ... snip ... ]