Notation of time
If the time is in Coordinated Universal Time (UTC), add a "Z" directly after the time without a space. "Z" is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".
UTC time is also known as "Zulu" time, since "Zulu" is the ICAO spelling alphabet code word for "Z".
Time zones are often represented by abbreviations such as "EST, WST, CST" but these are not part of the international time and date standard ISO 8601 and their use as sole designator for a time zone is not recommended. Such designations can be ambiguous. For example, "ECT", could be interpreted as "Eastern Caribbean Time" (UTC−4h), "Ecuador Time" (UTC−5h) or "European Central Time" (UTC+1h).
A time zone is a geographical region where just about everybody observes the same standard time.
A time offset is an amount of time subtracted from or added to UTC to get the current civil time - whether it's standard time or Daylight saving time.
In any particular time zone residents either observe standard time all year round (as in Russia or South Africa) OR they observe standard time in winter and daylight time in summer.
Conversion between time zones obeys the relationship
"time in zone A" − "UTC offset for zone A" = "time in zone B" − "UTC offset for zone B",
in which each side of the equation is equivalent to UTC. (The more familiar term "UTC offset" is used here rather than the term "zone designator" used by the standard.)
The conversion equation can be rearranged to
"time in zone B" = "time in zone A" − "UTC offset for zone A" + "UTC offset for zone B".
For example, what time is it in Los Angeles (PST, UTC offset= −08) when the New York Stock Exchange opens at 09:30 (EST, −05)?
time in Los Angeles = 09:30 − (−05:00) + (−08:00) = 06:30.
In Delhi (IST, UTC offset= +5:30), the New York Stock Exchange opens at
time in Delhi = 09:30 − (−05:00) + (+5:30) = 20:00.
These calculations become more complicated near a daylight saving boundary (because the UTC offset for zone X is a function of the UTC time).
The table "Time of day by zone" gives an overview on the time relations between different zones.
Since the 1920s a nautical standard time system has been in operation for ships on the high seas. Nautical time zones are an ideal form of the terrestrial time zone system. Under the system, a time change of one hour is required for each change of longitude by 15°. The 15° gore that is offset from GMT or UT1 (not UTC) by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from GMT by ±12 hours. A nautical date line is implied but not explicitly drawn on time zone maps. It follows the 180th meridian except where it is interrupted by territorial waters adjacent to land, forming gaps: it is a pole-to-pole dashed line.
A ship within the territorial waters of any nation would use that nation's standard time, but would revert to nautical standard time upon leaving its territorial waters. The captain is permitted to change the ship's clocks at a time of the captain's choice following the ship's entry into another time zone. The captain often chooses midnight. Ships going in shuttle traffic over a time zone border often keep the same time zone all the time, to avoid confusion about work, meal, and shop opening hours. Still the time table for port calls must follow the land time zone.