As Reggie Ray mentionned, it is stored in the DB (specifically the users table). This recent article although not related to your question, explains more about human dates versus unix time. Have a read of the last post, which also contains a link to a public time converter.
That said, you also might want to take a look at this mod located here:
Hope this helps,
NOTE: If you have any difficulties installing or operating this mod, in the event you decide to try it, you must seek support in it's support thread. Link provided above.