The Olympic Stadium, Stratford - 16th September 2014
The earth’s spin is not constant. It speeds up and slows down as mantle movements, tides and ice melts shift its mass around unpredictably. It's hardly enough to notice, but every few years there has to be a 'leap second' - a minute with 61 or 59 seconds in it - to correct for this.
It's not done to a fixed timetable. Instead, the International Earth Rotation and Reference Systems Service measures the earth's spin by astronomical observation and occasionally decrees that there will be a leap second. We get six months notice, but that's all.
No other watchmaker has timepieces accurate enough to worry about this. We have to. Just so long as you connect your watch to your smartphone every few months, the phone will tell it when there will be a leap second adjustment, just as it tells it about the next daylight saving adjustment.
If we're starting to sound a bit obsessed with accuracy, it's because we are. It's what makes us number one.