Nice, I love me a classic platformer! What can I say. The movement is snappy. It could be better with coyote time and jump buffers like YoloArts mentioned but since the jumps aren't that tight its perfectly fine. The gravity reverse and jump platform mechanics are oldies but goodies and they are implemented well. I really liked that you managed to make different themed worlds with their own music track and tileset.
The timer felt a bit unnecessary. If that ever reset me back to the start of the level it would have felt like doing the level again for no reason. There were also some blind jumps like in the night world when I had to land in the correct spot after hitting a gravity reverse. This caused some frustrations and trial and error.
A satisfying ending screen to top it off and everying together makes for a great jam entry. Keep it up!