As Xander suggested, MEL wouyld be the way forward, but if this is out of your league then you can key frame, and its a simple math.

1 rotate = 360 degrees
1 hour = 60 mins
15 mins = 90 degrees
keyframe every 60 frames at 90 degrees except the last one where you keyframe at 359 to allow it to go back to 0 again.

That would take care of the minute hand. The hour hand would folow the same path, but would be keyframed only every 4 th keyframe on the top of the hour.

Really MEL would be the way ahead to be honest and you would need to start looking at the time variable.