My 2000 SE recently starting pinging really bad under load, so I checked the timing and it was at 20 BTDC. So I reset it to 15 BTDC. How did the timing get off by so much? Does this mean that the timing belt is worn and I need to replace it?

BTW it has 82000 miles.

Seems this isn't uncommon:
http://www.automotiveforums.com/vbulletin/t2610.html

Brent