California Code, Business and Professions Code - BPC § 2456.2

(a) The board shall notify in writing by certified mail, return receipt requested, any physician and surgeon who does not renew his or her license within 60 days from its date of expiration.

(b) Any licensee who does not renew his or her expired license on or before its date of expiration shall pay all the following fees:

(c) Notwithstanding any other provision of law, the renewal of any expired physician's and surgeon's license within six months from its date of expiration shall be retroactive to the date of expiration of that license.