An hour used to be longer in summer than in winter.
Until the late Middle Ages, Europeans divided the time between sunrise and sunset into twelve hours, however long the day was. A summer hour in Rome lasted a good 75 minutes, a winter hour only about 45. Only the mechanical clocks of the 14th century, ticking at a steady rate, established hours of equal length.
Useful: Even the unit of time you think in is an invention with a date.
Source: en.wikipedia.org