In the film Back to the Future the Doc creates a time machine that can travel to any time in the past or in the future. But if we see the control board we might notice a possible problem.
The year is represented by an LCD display with four numbers. If we try to go to any year after 9,999 or before -999 it will not be represented! If we try to write it, the numbers will actually be written in the hour box instead.
An "overflow" happens when we want to represent a number that is too big or too small for the space it has available.