Well, I can't say what causes issues with IE8 (and cannot test under IE right now) but...
I wouldn't do it that way anyway.
What I would do is: Have the JS to do an AJAX call to the PHP page, and have PHP to return all you need to construct your timer - but - I would construct the timer on the JS side and not on the PHP side.
Hope this makes sense...
Chuck Norris counted to infinity.