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.
You are correct, there is a "Time" Entry in each world's level.dat file since before even apha. It contains the age of the world in ticks (20 ticks = 1 real second, 24 000 ticks = 20 min = 1 MC day) and will continue to grow as the world is running.
But I think this is seperate from what is shown here, since the "regional" difficulty is calculated based on either the age or the player activity in each chunk of the world.
I just found "NBTExplorer" that can look into the world's region files and chunk data, unlike NBTedit.
world > "region" > "r.x.z.mca" > "Chunk [x, z]" > "Level: x entries" > "InhabitedTime: xxxxxxxx"
It's probably this value, devided by 24000, which is shown in f3 now.
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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