It's Jayce language- If you are doing something to today then you will be doing it "inside of today." If you did something yesterday then you were doing that something "inside of yesterday" If you are going to do something tomorrow- you will do it "inside of the next day"- which is short for tomorrow!
I love it when he asks "What will we be doing inside of today?" It reminds me that I have this day- a full 24 hours- to do something. Its a funny little saying, but really makes me want to do something great "Inside of everyday"