The crop timer should never change suddenly if the game is running on a computer with a stable system clock that hasn't been adjusted. The most frequent cause of a crop time that long is that there has been a change in the computer's system clock. Even it you didn't adjust the system clock, it can be inadvertently changed if you double-click that little Windows clock in the bottom right corner to check dates and times (a lot of people do that without realizing that it changes anything on their computer). Also, Windows has the ability to set the computer's clock using an Internet time reference, so if the clock was inaccurate the resulting time correction could affect the game.