As a manufacturer of NTP time servers, synchronizing computer networks and keeping them accurate to within a few milliseconds of international UTC time (Coordinated Universal Time), we often think we can keep pretty good track of time.
Time, however, is quit elusive and is not the fixed entity we often assume it is, indeed time, and the time told on Earth is not constant and is affected by all sorts of things.
Since Einstein’s famous equation, E=MC2 it has been acknowledged that time is not constant, and that the only constant in the universe is the maximum velocity of light. Time, as Einstein discovered, is affected by gravity, making the time on Earth run slightly slower than time in deep space, likewise, on planetary bodies with a larger mass than Earth, time runs even slower.
Time slows down when you approach very fast speeds too. The property of time, known as time dilation, was discovered by Einstein and means that at close to the speed of light, time almost stands still (and makes interstellar travel a possibility for science fiction writers).
Generally, living on Earth, these differences in time are not felt, and indeed the slowing of time caused by Earth’s gravity is so minute, highly precise atomic clocks are required to measure it.
However, the time we use to govern our lives is also affected by other factors. Since humans first evolved, we have been used to a day lasting just over 24 hours. However, the length of a day on Earth is not fixed, and has been changing for the last few billion years.
Each day on Earth differs from the previous to the next one. Often these differences are minute, but year on year, the changes add up as the affect of the moon’s gravity and tidal forces act as a brake on the Earth’s spin.
To cope with this, the global timescale UTC (Coordinated Universal Time) has to be adjusted to prevent the day from drifting out of sync (and we end up with noon at night and midnight during the day—although at the current slowing of the Earth, this would take many thousands of years).
The adjustment in our time is known as leap seconds which are added either once or twice a year to UTC. Anybody using a NTP time server (Network Time Protocol) to synchronise their computer network too, needn’t worry, however, as NTP servers will automatically account for these changes.