Dob validation

I wrote rspec tests when I submitted the pr for dob_validation. I can remember writing them. Somehow, though, they didn't come through to the pr. I've long since deleted the branch so can't see what went wrong. 
Here is a small pr to put the tests back in to the model specs.
Not sure if this is clearer or not, but an alternative could be `(1.year + 1.day).ago`