Earth is split up into a multiple time zones. Most timezones are exactly one hour apart, and by convention compute their local time as an offset from UTC or GMT. In many locations, these offsets vary twice yearly due to DST transitions.
Every country may have a particular standard time zone though, if it is more convenient for its world cooperation. The standard time zone is a region where the local or national authorities unify the time for a particular goal. The time zones obey specific rules referring to longitude.
