Need Help with a Continuous Counter

Hi, I’m looking to create a continuous counter and I’m having some trouble with the syntax of the formula. Basically I have a “Start Date & Time” and a “Start Number” I also Have a PHP scrip that will bring in the “Current Time” and a “Rate of Change” (.62/sec).

This is what I think the formula might look like (not in code):

Current Time - Start Time = # of seconds
(# of seconds x Rate of Change) + Original Number = Final Number

Thank you for any help.