John lives in Pensacola, Fla., which is in the Central time zone. Mary lives in Ontario, Ore., which is in the Mountain time zone.

At 2 a.m. on the first Sunday in November, John sets his clock back 1 hour to standard time. One hour will pass before Mary too sets her clock back. So for one hour each year, they observe the same time.

From Frank Morgan, The Math Chat Book, 2000.

See also Four-Dimensional Basketball, Pop Fly, and A Freak of Navigation.