I've read (tried to anyway ) several articles on wikipedia about option pricing and I want to know how to calculate past time value. Here's what I want to do: At any time, take an option and somehow calculate what it's price "was" X-number of days ago. Is this possible? I already know that the intrinsic value X days ago is the stock price minus the strike, so I assume I want a theta formula of some sort. Any help would be great, I just want to say first that if you have a formula could you please write it with general math in mind (+ - * / sqrt). Trying to read wikipedia left me scratching my head. Thanks in advance.