Very charming game with a great use of the theme! Simply lacking in content
There isn't much I can say on this game, as there simply isn't enough of it to critique.
Most criticisms of the game, are simply the lack of features, rather than flaws with existing ones.
Try adding sound/music to the game, as without it the game feels underwhelming. Try more unique obstacle types in the game, such as the basic moving obstacles or projectile firing types.
Visually, I think changing the grass colors or lighting when traversing between different time periods, would add a lot to the feel of travelling between wholly different periods in time.
I think with a bit more time and work, this could be a very cool experience, as the concept is fantastic!