Cute idea... good way to get them to learn square roots without them knowing it because they know the numbers under the post its! It is very inexpensive and not at all time consuming to make. The only problem I can think of with this would be the postit notes loosing their ability to stick onto the clock's face.
50 Things I'm Not Allowed To Do At Hogwarts. These Are Bloody Brilliant.
graphing radical functions (square root and cube root functions using transformations) and solving radical equations from the miss jude math! TPT shop This resource is free, but all purchases fund scholarships for youth in need!