The summary of the bug is that when you play a world in the latest snapshot the data that is was played goes to 1/1/70 which is really far off (the date on my computer is correct) this puts your world at the bottom of your world list

To recreate the bug you have to:

1) Make a world
2) Play the world
3) Go back to the menu
4) Singleplayer
5) Look for the world at the bottom of the list and it will say someting along the lines of 1/1/70