Let's practice this during the daytime so it makes sense before you try to impress your friends. Get out a pencil and a sheet of paper.
First, you'll need to find Polaris. Draw a dot in the middle of your paper. Instant star.
Draw a BIG circle around the dot. Now imagine the mirror-image of your kitchen clock imposed over Polaris in that circle. This means 1 is where the 11 usually is, and the 3 is where the 9 normally goes. Draw all twelve digits inside that circle around Polaris.
Now imagine you are looking up at the dark night sky. Imagine yourself finding Polaris. Stare at it. Draw an imaginary line out through Dubhe and Merak (the pointer stars in the Big Dipper that lead you to Polaris). This straight line is the hour hand of the clock (there is no minute hand). This hour hand makes a complete circle around the North Star every 24 hours.
At midnight on March 1, the hour hand points directly to midnight (straight up). When this hour hand moves, it moves counterclockwise. And every 5-minute mark on regular clocks (this is the 1, the 2, the 3, the 6 at the bottom …) represent two hours. So if the hour hand points straight to the 3, (normally the 9 position on your kitchen clock), then it is 6 hours past midnight on March 1. Got that so far? Good. Let's make it harder.
Subtract 2 hours for every month past March 1. For example, if it is May 1, subtract 4 hours. (Add one hour for daylight savings time.)
Let's try an example. Imagine it is June 1. Imagine the star-clock is one forth the way past the 1 on your mirrored-kitchen-clock paper diagram. It currently reads 2:30 am (remember it is two hours between the 1 and 2 on the clock). Now subtract 2 hours for each month past March 1. Since it's June 1, subtract 6 hours. Add one hour for daylight savings time, and you now get 9:30 pm. Cool!
With practice, you will always be able to tell the time by the stars in a matter of seconds (trust me). And you'll no longer wonder "What time IS it?" And be scolded by fellow astronomers as you pull out a flashlight to glance at your watch (albeit red). And it is a nifty ooooh-ahhhh trick with kids-at-heart everywhere.