You'd need some way of syncing real-world time with a source outside of the game. There are 2 plugins that get the system date an time on the user's computer:ClockSystem Date and Time
If you're concerned that the user would cheat by messing with their system clock, you can sync with an external source to get the correct time, but this is a bit more complex. If you have your own server, you can get the time by setting up a simple PHP script. Then, within the game, ajax request the php page.
Once you have a method of getting the current time, all you'd need to do is record the time the item is acquired, and destroy the item if more than an hour or more has passed.