The right time
In some airbases, the military in on one side of the field and the 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: "Wo is calling?"
The aircraft replied: "What difference does it make?"
The tower replied: "It makes a lot of difference. If you are a United Airlines Flight, it's 3 o'clock. If you are an Air Force Flight, it's 15.00. If you are a Navy Flight, it's 6 bells. If you are an Army Flight, the big hand in on the 12 and the little hand is on the 3. If you are a Marine Corps Flight, it's Thursday afternoon.
http://www.321tigers.org/Bonerbook.html#b14