"[...] Everything must be created during the time frame of the game jam.
Using assets that are not your own is OK! Be sure to properly credit people."

So, I can only use assets created out of the time period if they are owned by other people?

Good question! You can use assets that were created (by other people) from any time. So, for example: say you find free models in Unity that were made by someone else - totally OK to use them, no matter when they were made.