Decent game for a 1st entry in a jam. I try to give as much feedback as I can to lots of games, so here are some aspects that can be improved:
1) The background is simple but works. If you want to make it better then I recommend learning how to make a parallax background. 2) The countdown could be implemented more into the gameplay. Right now it is just a hazard. Maybe you want to add time clocks that add more time or reward players for taking less time or more risks. 3) A few more levels would be nice so that the game doesn't end after 2 screens. 4) since this is a platformer, I would highly recommend to make it as fair as possible (fair ≠ easy). You already have good checkpoints. I would also add the usual stuff like jump buffering or small collision boxes.
Overall I think you have made a 1st game that is a lot better than most other 1st games. Good job!