Author Tim Harford, who wrote The Logic of Life and a few other books I've liked, yesterday published an explanation of what telling time is all about:
Water clocks appear in civilisations from ancient Egypt to medieval Persia. Others kept time from marks on candles. But even the most accurate devices might wander by 15 minutes a day. This didn't matter to a monk wanting to know when to pray.
But there was one increasingly important area of life where the inability to keep accurate time was of huge economic significance: sailing.
By observing the angle of the Sun, sailors could calculate their latitude - where they were from north to south. But their longitude - where they were from east to west - had to be guessed.
Mistakes could - and frequently did - lead to ships hitting land hundreds of miles away from where navigators thought they were, sometimes disastrously.
How could accurate timekeeping help? If you knew when it was midday at Greenwich Observatory - or any other reference point - you could observe the Sun, calculate the time difference, and work out the distance.
But does anybody really know what time it is?