Is your system time/location data for your Xbox set correctly? Perhaps when the game starts it does a check for current time as compared to what time the game thinks completion is due, and if some details don't match, then the timer gets reset?
its awhile after you posted but was there anything you could do to fix this bug im having it now and its much more than 1300 its in the 10s of thousands
I encountered this glitch myself today on my Xbox 360. I tried going into the settings menu on the Xbox menu itself and I did two things. First, I went to Initial Setup and re-applied the time zone information, language etc. Then I went to clock and found that the Xbox chromometer got sent back by 10 years! I corrected the time and when I loaded my game the glitch was gone. So one of those worked although I'm really certain it was fixing the clock that did the trick.