400 Years is an online puzzle/simulation where the player controls some sort of being that senses a disaster is about to happen and they have 400 years to prevent it via speeding up time and helping people out so they will, in turn, help him out during the game.
How To Find The Easter EggEdit
Make your way through the entire game, but don't enter the volcano. Go back a few screens to where the last chestnuts were (you will probably have to speed up time to regrow the tree to climb it again), pick some up, then go past the volcano. When you come to the wall you can't climb, plant the chestnuts, speed up time to grow them into a tree, then climb the tree to get to the next area. There you will find a gravestone with a message on it.
The Easter Egg Edit
Adventure The message on the gravestone reads as follows,
'Here lies a traveler between worlds, Who's name is unknown. May his curious soul find a way home, Like you have found his resting place.'
The only theorized meaning that has been discussed is that the 'traveler between worlds' is the player. They were curious enough to find the gravestone, and must now find a way 'home,' meaning the end of the game.