Advanced countdown timer

I have searched the web for hours and have not found anything to even help me start on my problem...

I'm not even sure of the script I'm looking for...

I have very little experience in scripting.

What I want is to have a countdown timer that will countdown till the end of an event, but will only count down while the event is in progress at my Local Time. (eg. hours left of grade 10: will only countdown when I'm actually at school)

Anyhelp would be greatly apreacheated. Even a gentle prod in the right direction.

Use php to compare the time using the date() function.

If the php returns true, you then echo the javascript countdown.

http://www.google.ca/search?hl=en&q=javascript+countdown&meta=