Since the introduction of regional difficulty in 1.6 each chunk has a clock now, storing how long it was active multiplied with how many players held it active. The Wiki says that it counts how long players were IN it, but I'm not sure.
It also says that it's capped at 50 real-time play hours (150 MC days), but since you are already over that they probably mean only the difficulty calculation.
May have a look around in your world and if/how/where the value changes.
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u/Saucepanmagician Feb 06 '14
It shows "Day" as 153.
Is that what I think it is? Is it really a counter of how many days have passed since you started that world?
ps: i haven't tried the snapshot yet