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 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’re a United Airlines Flight, it’s 3 o’clock.
If you’re an Air Force flight, it’s 1500.
If you’re a Navy flight, it’s 6 bells.
If you’re an Army flight, the big hand is on the 12 and the little hand is on the 3.
If you’re a Marine Corps flight, it’s Thursday afternoon.”