If you are wondering how long is 100 days in Minecraft, the answer is around 33 hours of real-time.
So if you are trying to beat the Passing of Time achievement for the game, we highly recommend just playing the game for that total amount of time until the achievement unlocks for you.
You can also complete this achievement really efficiently by AFKing in the game on peaceful mode for a few hours each day. However, if you do this afk method, make sure to plug your controller and make sure the game stays active for the allotted amount of time.
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It may take a little longer, but simply playing the game will be enough and probably easier than leaving your game running all day to complete the achievement.
For other achievements like the Free Driver achievement, you will need to stay underwater for two minutes, and the Castaway achievement, you will need to eat kelp for nothing but two days.
If you want to complete this achievement, we recommend doing it while you are trying to get the Passing of Time achievement. But for the Free Driver achievement, we recommend getting some potions of water breathing.
Check out some of our other awesome Minecraft guides!