When I start a survival world, it works fine. When I close and reopen Minecraft Pocket Edition, the world seems to get corrupted. The following things occur:

1. The world creation date is changed to December 31st, 1969
2. The world's title is removed
3. The world is changed to Creative Mode

When I try to open the corrupted world, Minecraft crashes.

There are no mods on the game, I haven't used external editors.