Your second PEN isn’t working but it appears the one you linked in the OP is behaving properly.
I notice you’re already checking the date as a comparison in your setInterval function: bravo!
Some general info on why this is good:
SetInterval is not guaranteed to execute your callback functions exactly at the set time interval – it’s just guaranteed to be at least X milliseconds later (where x is the delay.) There is a chance of slight variance.
http://ejohn.org/blog/how-javascript-timers-work/ (older, but good)
http://stackoverflow.com/questions/11042510/javascript-duration-of-animation-isnt-exact (see accepted answer)
One strategy for circumventing this annoyance is to check the date in your setInterval and compare it to when you started counting and adjust accordingly. This ensures your countdown will be completely accurate.
http://www.andrewduthie.com/post/a-self-correcting-setinterval-alternative/
