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 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."