That's a classic.
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, its 3PM.
If it is Air Force, its 1500 hours.
If it is a Navy aircraft, its 6 bells.
If it is an Army aircraft, Mickey's big hand is on the 12 and Mickey's little hand is on the 3.
If it is a Marine Corps aircraft, its Thursday afternoon."