What does the hourglass do?

When you have generators with a cooldown time, like the orange trees or for special decorations (like the toy blocks, or aquarium fish) the hourglasses take time off how long until those generators can produce again. For instance, a Potted Orange Tree (level 8) usually takes 2-3 hours to respawn oranges, but if you use Green Hourglass, that cooldown timer will be reduced by 40 minutes.

If you get the special hourglass that eliminates cooldown timers for 15 minutes, and a couple red energy bottles, you can get really far ahead on the special decoration quests.