inhabitedTime

The cumulative number of ticks players have been in this chunk.

Note that this value increases faster when more players are in the chunk.

This is used for regional difficulty. It increases the chances of mobs spawning with equipment, the chances of that equipment having enchantments, the chances of spiders having potion effects, the chances of mobs having the ability to pick up dropped items, and the chances of zombies having the ability to spawn other zombies when attacked.

Also note that regional difficulty is capped when this value reaches 3600000, meaning that none of the above will increase past that point.

See here for more details.