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.
Math Coach's Corner: Using Math Stretches for a Daily Warm-Up. If you are a Laney Sammons fan, then you probably already know about Math Stretches. Part of the Guided Math framework is Morning Math Warm-Ups and Calendar Activities. Sammons explains that just like athletes warm up before a strenuous workout, mathematicians need to have a warm-up to get their brains focused on math.