Oscar van Vlijmen wrote:

[...] Gwillim Law noted:

...

An Anglo-French Conference on Time-Keeping at Sea (June, 1917) agreed that legal time on the high seas would be zone time, i.e., the standard time at the nearest meridian that is a multiple of fifteen degrees.

[...] Theoretical timezones: Take [0;15W), [15;30W),..., [165;180W), [180E;165E),..., [15E;0E) Times in hours from UT. [ means including, ) means not including. 0-15E +1 [...]

Minor correction: what I was trying to express was that ships at sea should use UTC between 7.5 degrees E and 7.5 degrees W, UTC+1 between 7.5 degrees E and 22.5 degrees E, etc. Gwillim Law