Time Locked Puzzles and Proof of Work

So how might we a lock down a file for a given amount of time or make sure that someone does not get their present until their birthday? Well one way is to get others to do some work, that you know will be done within a certain amount of time. If I know it will take you one hour to cut the grass in your garden, before I give you your reward, I will hope you will not be able to do it quicker than one hour.