What is the Correct Time?

The Development of Time Scales From GMT to UTC and Their Implementation in Modern NTP Servers

Asking somebody the time may be one of today's most common questions but have you ever wondered where the time on our watches comes from?

Accurate clocks have only been around since the mid 17th century, before then, time was completely subjective. People would use the celestial bodies as a time reference such as noon (when the sun was highest) and midnight (when the moon is at its highest) and also dawn and dusk. Often lengths of time were referred to in comparison such as the time it would take a man to walk a mile.

Standard timescales did not exist until the 1840's when it became necessary during the height of the railway's popularity when a railway standard time for all England, Wales and Scotland replaced all the local timescales.

A few years later the Royal Observatory in Greenwich developed its own time scale. This was based on the sun and moon, with 12 o'clock (noon) being when the sun was over the Greenwich Meridian, they began transmitting this timescale using the telegraph and by 1855 most of Britain used GMT (Greenwich Mean Time) and it soon became a recognized time reference throughout the world.

However, it became apparent with the invention of atomic clocks that basing a time reference on the movement of the Earth was not accurate enough. In 1967 the second was defined by the oscillations of the caesium -133 atom (as used in atomic clocks) and provided the most accurate reference for time yet but attempts to couple GMT with this new definition proved unsatisfactory when it was discovered that the Earth slows (and speeds up) on its axis.

This variations in the rotation of the Earth meant a new timescale UTC, (Coordinated Universal Time) which made adjustments for this slowing adding (or subtracting) a second when ever necessary (failure to do so would mean eventually day would become night as time would slip, albeit in many millennia). This addition is known as a Leap Second.

UTC has become vital in allowing the global community to communicate with each other. UTC allows the world to synchronise to one time scale regardless of the time zone (UTC handles timezones with a + or minus such as UTC +5 or UTC -2)

UTC enables computers to synchronise together all over the world using NTP (Network Time Protocol). Without NTP it would be impossible to conduct time sensitive transactions such as buying an airline ticket or bidding on Ebay.

Most NTP time servers receive UTC time atomic clocks from either a broadcasted signal from a large physics laboratory or via the GPS network.