On some air bases the Air Force 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 it is 
an American Airlines Flight, it is 3 o'clock. If it is an 
Air Force, 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 afternoon. If it's National 
Guard, it's still a couple of hours until quitting time."