Custom Countdown Clock via Form - Help!

[evcruise]

I want to create a countdown clock that users of my forums can put in their siguature. I want to make it so that they can create it via a form and so that they can pick specific options that end up making the image custom for them. Here's an example of what I'm trying to do:

cruisecritic.com/vbulletin/countdown.cfm

I know how to create forms, but I don't know how to do the back end of it. Anyone know what I need to do to do this? Thanks so much!