So leap seconds - and leap years - are added as means of keeping our clocks (and calendars) in sync with the Earth and its seasons.
One theory is that the custom dates back to the 5th Century, when, legend has it, an Irish nun called St Bridget complained to St Patrick that women had to wait too long for their suitors to propose.
St Patrick then supposedly gave women the chance to ask the question every four years.
The tradition is not thought to have become commonplace until the 19th Century.
The system was tweaked, however, about 500 years later.
As an earth year is not exactly 365.25 days long Pope Gregory XIII's astronomers decided to lose three days every 400 years when they introduced the Gregorian calendar in 1582.
The maths has worked ever since but the system will need to be rethought in about 10,000 years' time.Perhaps mankind's robot overlords will think of something.All the other months in the Julian calendar have 30 or 31 days, but February lost out to the ego of Roman Emperor Caesar Augustus.Under his predecessor Julius Caesar, Feb had 30 days and the month named after him - July - had 31. When Caesar Augustus became Emperor he added two days to 'his' month to make August the same as July.So February lost out to August in the battle of the extra days.Leap years are also marked as a time for women to propose to men.