Nautical time is a maritime time standard established in the 1920s to allow ships on high seas to coordinate their local time with other ships, consistent with a long nautical tradition of accurate celestial navigation. Nautical time divides the globe into 24 nautical time zones with hourly clock offsets, spaced at 15 degrees by longitudinal coordinate, with no political deviation. Nautical time keeping dates back to the early 20th century as a standard way to keep time at sea, although it largely only applied to military fleets pre–World War 2. This time-keeping method is only used for radio communications and to account for slight inaccuracies that using Greenwich Mean Time (GMT) may lead to during navigation of the high seas. It is typically only used for trans-oceanic travel, as captains will often not change the timekeeping for short distances such as channels or inland seas. History of nautical time Establishment 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. Letter suffixes 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 can be vocalized using the NATO phonetic alphabet which pronounces the letter Z as Zulu, leading to the use of the term "Zulu Time" for Greenwich Mean Time, or UT1 from January 1, 1972 onward. Zone Z runs from 7°30′W to 7°30′E longitude, while zone A runs from 7°30′E to 22°30′E longitude, etc. These nautical letters have been added to some time zone maps, like the World Time Zone Map published by Her Majesty's Nautical Almanac Office (NAO), which extended the letters by adding an asterisk (*), a dagger (†) or a dot (•) for areas that do not use a nautical time zone (areas that have a half-hour or quarter-hour offset, and areas that have an offset greater than 12 hours), and a section sign