The Anglo-French Conference on Time-keeping at Sea was held in London in June 1917. The Conference established the nautical date line and adopted an ideal form of the terrestrial time zone system for use at sea. It recommended that time changes required by changes of longitude be made in one-hour steps. This recommendation was adopted between 1920 and 1925 by all major fleets, including British, French and American. The rules applied to almost all naval ships and to many non-naval ships. Nevertheless, up to the Second World War, the old practice of keeping local apparent time prevailed on many independent merchant ships.