This Countdown program from Matt's Script Archive will display the number of years, months, days, hours, minutes and seconds until a specific event occurs. You can also allow your users to type in a date and find out how long it is until then.
The Readme files at Matt's Script Archive contain detailed installation, configuration and usage instructions that will take you step by step from putting the scripts on your system to using them in your web pages. You should read these through and keep them by you when setting up or installing any MSA script. It is probably not a bad idea to print them out. Each compressed script package contains a plain text version of the Readme file, however the links below will take you to their HTML counterparts.