On some air bases, the military uses one side of the field and civilian aircraft use the other side, with the tower in the middle serving both. One day, at one of these fields, a call from an aircraft called in asking, "Hey, Tower, what time is it?"
The tower answered, "Who is calling?"
The aircraft answered, "What difference does it make?"
The tower responded with, "It makes a lot of difference. If you are a civilian aircraft, it's three o'clock; if you're an Army aircraft, it's 1500 hours; if you're a Navy aircraft, it's 3 bells; if you're an Air Force aircraft, the big hand is on 12 and the little hand is on 3; and if you're a Marine aircraft, it's Thursday afternoon and 120 minutes 'til Happy Hour."