Standard time is the synchronization of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time.
The adoption of standard time, because of the inseparable correspondence between longitude and time, solidified the concept of halving the globe into the Eastern Hemisphere and the Western Hemisphere, with one Prime Meridian replacing the various prime meridians that had previously been used.