On some air bases, the military is on one side of the field and civilian aircraft use the other side of the field, with the control tower in the middle.
One day, on just such a field, the tower received a call from 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 you are a commercial airlines flight, it is 3 o'clock.
If you are an Air Force aircraft, it is 1500 hours.
If you are a Navy aircraft, it is 6 bells.
If you are an Army aircraft, the big hand is on the 12 and the little hand is on the 3.
If you are a Marine Corps aircraft, it's Thursday afternoon.
If you are in the National Guard, it's still a couple of hours until quitting time."