I think you will have to delete your current countdown and create a new one. Or, if you feel comfortable, go into your profile signature and see if you can find where the date is listed and change that. If that doesn't work then I would go with the delete and make a new one idea!
Click on User Cp (at top of the page) and then on Edit Profile. You will see your signature in a box on that page. Scroll down through your signature until you see the date of your trip. Change what you need to in the date but do not change any other text. Once you click on the button to save your changes your countdown clock should reset.