Standard Time is the harmony of clocks in various geographical locations within a time zone to a common time standard. This is based on the meridian at the centre of the time zone. This concept of Standard Time led to the halving of the globe into an eastern and western hemisphere, with one Prime Meridian and also its opposite International Dateline. This one Prime Meridian replaces the various Prime Meridians which had earlier been used.
In December 11, 1847, the British railways first used a standardized time system, when they switched from local mean time, which varied from place to place, to Greenwich Mean Time (GMT).
In United States, the Pennsylvania Railroad used the “Allegheny Time” system. This time system is an astronomical timekeeping service developed by Samuel Pierport Langley in Allegheny Observatory in Pittsburgh. In Netherlands, the Amsterdam Time or Dutch Time was introduced in May 1909.
Time Zone is a region which has a consistent standard time for legal, commercial, and social objectives. Prior to 1972, all time zones were stated as an offset from GMT, which was the mean solar time at the meridian crossing the Royal Observatory in Greenwich, London. Later, all official time services made the radio time signals in harmony with Coordinated Universal Time (UTC), a kind of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT, now known as UT1 (Universal Time 1). Many nations have started legally defining their standard time relative to UTC. Still some countries such as United Kingdom legally refer to GMT. UTC is also known as Zulu time, and is used throughout the globe by astronomers and other people who need to organize some event setting the time zones amongst various countries.
It is known that time zones are based on GMT, which is the mean solar time at longitude zero degree (the Prime Meridian). Now the time derived from atomic clocks is adjusted to remain within a second of UT1, as the rate of rotation of the Earth is not steady. The length of the second in both GMT and atomic time was balanced. The readings of atomic clocks are equated to provide a consistent time scale.