Basic concepts related to time
Earth is split up into a number of timezones. 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.
GMT and UTC
The implementation of Greenwich Mean Time was the first step to determine the time zone of other countries in regard to GMT+0, while the concept of Coordinated Universal Time (UTC) was designed to provide a more accurate timekeeping system. Nevertheless, both of these time standards are widely used in the world for a similar purpose of time coordination.
The differences in the terminology of GMT and UTC still create confusion in international cooperation. Even though UTC was introduced as a more accurate time standard, the occurrence of the leap seconds demonstrated the flaws for the universal time synchronisation.
These abbreviations are used to identify the time in the 12-hour clock format instead of using 24 hours. Considering that day has two cycles, AM and PM denote whether it is before or after the middle of a day.
AM/PM within the 12-hour time format is not effective way to indicate the precise time. There might be confusion regarding the time of a day as well as it can mislead in the date when midnight is not properly indicated. Considering that legal and other formal standards require the use of 24-hour convention.
Daylight Saving Time
The concept of Daylight Saving Time (DST) was proposed by Benjamin Franklin and refers to advancing the standard time within the region or zone by one hour to enhance the duration of daytime. Typically clocks are adjusted forward one hour in spring and backwards in autumn.
Consequently, a location observing UTC+3 during standard time is at UTC+4 during DST; a location at UTC−6 during standard time is at UTC−5 during DST. Clock shifts are usually scheduled on weekends to lessen disruption of weekday schedules.