You need a small screwdriver to change the time on the standard Hilton clocks - the time settings are hidden underneath a panel.

I believe it's the panel with the holes for the 5 audio control buttons (e.g. rock, pop, mp3 etc...) and it's locked in place by a small screw.

Handily though - you'll often find someone has removed the screw so the buttons are easily accessible. I like this as I'm a little OCD about clocks not set to the correct time...