The way a computer deals with time is totally different to the ways humans perceive it. We arrange time into seconds, minutes, hours, days, weeks, months and years, while computers on the other hand arrange time as a single number representing the seconds that have passed from a single point in time, known as the prime epoch.
Most computers use NTP (Network Time Protocol) to deal with time and on networks many are synchronised using a dedicated NTP time server. NTP knows nothing about days, years or centuries, only the seconds from the prime epoch. This prime epoch is set (for most systems) at midnight at the turn of the century twentieth century that for a human would be recorded as something like: 00:00 – 01,01,1900.
Computers, however, count time as the number of seconds past this point. If a computer was around in 1900 its timestamp on midnight January 1 would be 0 while in 1972 at the same date the timestamp would be 2,272,060,800, which represents the number of seconds since 1900.
The timestamps restart every 136 years with the next wrap around due in 2036, this has caused uneasiness amongst some who fear a Millennium Bug type scenario, although most doubt such events would occur, however, when a wrap-around of the timestamp does happen an era integer will be added (+1), to allow computers to deal with time spans that cover more than one wrap-around. If computers and NTP need to deal with time that spans before the prime epoch a negative integer is used (for the year 1500 a -3 will be used to represent three cycles of 136 years).
Timestamps are used in virtually every transaction that modern computers are tasked to do such as sending emails, debugging and programming. Because time is linear, a computer knows that each timestamp is always greater than the previous one and therefore computers and NTP find it difficult to deal with inaccuracies in time, particularly when time suddenly appears to go backwards.
This can happen if computers are not synchronised to the same time. If an email is sent to a machine with a slower clock, it appears to the computer to have been received before it has been sent. Lack of synchronisation can serious problems and can even leave a system vulnerable to malicious attacks and even fraud.
Because of this, most computer networks are synchronised to UTC (Coordinated Universal Time). UTC is a global timescale and the same for everybody worldwide it is based on the time told by atomic clocks which are highly accurate, neither gaining nor losing a second in millions of years.
Most computer networks use a dedicated NTP time server to receive a UTC time to synchronise their computers too. UTC is available from across the Internet (although unsecured), via the GPS network (Global Positioning System), or by receiving national time and frequency broadcasts via long wave.
NTP synchronises a computer by checking the received UTC time and adding to or holding a computer’s timestamp until it perfectly matches UTC. By using a dedicated NTP time server UTC can be maintained on a network to a few milliseconds of UTC time.