I just looked at the code for my ticker and I see a number in there that says "20060816" (we are arriving 8/16/06), so you might try changing that number to your new date. If that doesn't work, I'd just delete the old one and make a new one.
Click on User CP on the left hand side of the toolbar above the threads. Once you get there, click on Edit Signature. You can adjust the dates on the one you already have or delete it and add a new one.