I like the art and think the music is quite fitting, but I think it would be nice if there was some backround art/maybe more frequent upgrades/more upgrade variety (like shooting mutiple bullets for example), some more effects could also be nice and I think the enemies should also spawn farther away from the player. I would also change the movement a bit, so that you still move a little after releasing the movement buttons/maybe adding some recoil and also adding knockback to the enemies. I do like the implementation of the special object, but I am usually not really a fan of having the theme implemenation just be lorewise, maybe there could be some sort of gamplay feature revolving around time travel, but if it is lore only I think it should be at least implemented in the game a little bit (maybe with a cutscene/lore tablet at the start of the game) (if there is some sort of lore implemented into the game I might just not have gotten to it), becuase very often when you just add the theme as some sort of lore to the game it can seem like you just made a game not really relevant to the theme and then thought of something that could connect it (I am not saying that you did that though, it just makes it seem like that)