Nautical timekeeping 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.
These zones were adopted by all major fleets between 1920 and 1925 but not by many independent merchant ships until World War II.
UTC, atomic time at Greenwich, makes these adjustments on a coarser granularity than GMT.
On short passages the captain may not adjust clocks at all, even if they pass through different time zones, for example between the UK and continental Europe.
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.