On some air bases the Air Force is on one side of the field and in the civilian aircraft use the other side of the field, with a the control tower in the middle. One day the tower received a call from a an aircraft asking, "What time is it?"
The tower responded, "Who is calling?"
The aircraft replied, "What difference does it make?"
The tower replied, "It makes a lot of difference. If it is an American Airlines flight, it is 3 o'clock. If it is an Air Force plane, it is 1500 hours. If it is a Navy aircraft, it is 6 bells. If it is an Army aircraft, the big hand is on the 12 and the little hand is on the 3. If it is a Marine Corps aircraft, it's Thursday."