The establishment of nautical standard times, nautical standard time zones and the nautical date line were recommended by the Anglo-French Conference on Time-keeping at Sea in 1917. The Conference recommended that the standard apply to all ships, both military and civilian. These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.
The nautica time zone system is an ideal form of the terrestrial time zone system for use on high seas. Under the system time changes are required for changes of longitude in one-hour steps. The one-hour step corresponds to a time zone width of 15° longitude. 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.[1]
Time on a ship's clocks and in a ship's log had to be stated along with a "zone description", which was the number of hours to be added to zone time to obtain GMT, hence zero in the Greenwich time zone, with negative numbers from −1 to −12 for time zones to the east and positive numbers from +1 to +12 to the west (hours, minutes, and seconds for nations without an hourly offset). These signs are to those given below because ships must obtain GMT from zone time, not zone time from GMT. All zones were pole-to-pole staves 15° wide, except −12 and +12 which were each 7.5° wide, with the 180° meridian separating them. Unlike the zig-zagging land-based International Date Line, the nautical International Date Line follows 180° except where it is interrupted by territorial waters and the lands they border, including islands.
Around 1950, a letter suffix was added to the zone description, assigning Z to the zero zone, and A–M (except J) to the east and N–Y to the west (J may be assigned to local time in non-nautical applications; zones M and Y have the same clock time but differ by 24 hours: a full day). These were to be vocalized using a phonetic alphabet which pronounces the letter Z as Zulu, leading sometimes to the use of the term "Zulu Time". The Greenwich time zone runs from 7.5°W to 7.5°E longitude, while zone A runs from 7.5°E to 22.5°E longitude, etc.
These nautical letters have been added to some time zone maps, like the World Time Zone Map by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*) or dagger (†) for areas that do not use a nautical time zone, and a section sign (§) for areas that do not have a legal standard time (the Greenland ice sheet and all of Antarctica. The United Kingdom specifies UTC−3 for the Antarctic Peninsula, but no other country recognizes that).
In maritime usage, GMT retains its historical meaning of UT1, the mean solar time at Greenwich. UTC, atomic time at Greenwich, is too inaccurate, differing by as much as 0.9 s from UT1, creating an error of 1/4 of a minute of longitude at all latitudes and which is 1/4 of a nautical mile or 0.4 km at the equator but less at higher latitudes, varying by the cosine of the latitude. However, DUT can be added to UTC to correct it to within 50 ms of UT1, reducing the error to only 20 m.
A ship is required to adopt the standard time of a country when it is in its territorial waters, but must revert to nautical time as soon as it leaves territorial waters.
In reality nautical times are used only for radio communication etc. Internally on the ship, e.g. for work and meal hours, the ship may use a suitable time of its own choosing. The captain is permitted to change his ship's clocks at a time of his choice following his ship's entry into another time zone — he often chose midnight. Long distance going ships change time zone onboard at suitable times. Ships on short distance journeys do not change time zone at all, even if they go between different time zones, like between the UK and continental Europe. Passenger ships often use both time zones on signs. In time tables and communication with land, the land time zone has to be used.
Before 1920, all ships kept local apparent time on the high seas by setting their clocks at night or at the morning sight so that, given the ship's speed and direction, it would be 12 o'clock when the sun crossed the ship's meridian. The local apparent noon is 12 noon.