In relation to Greenwich Mean Time
Unlike today, Great Britain , France and Belgium all operated at the same time during the Great War. (The latter two countries are of course now one hour ahead of the United Kingdom). They were set at GMT. France had only adopted this as recently as 1911.
German time was one hour ahead of GMT. In other words, one hour ahead of the British, French and Belgian time.
French territory occupied by German forces in 1914 had to adopt German time.
The British Summer Time Act of 1916 was passed to enable daylight saving for production and agricultural purposes. The time in use in Britain was advanced by one hour ahead of GMT at 2am on 21 May and remained so until reverted back on 1 October. This change did not affect the British force engaged in France. This cycle continued in the later years.
Germany also adopted daylight saving between 11pm on 30 April (when time advanced to 00am on 1 May) and 1am on October 1916 (when it re-set to 00am). Throughout this period, time in use was at GMT plus two hours.
Adoption of daylight saving time in France and on the Western Front
France also also adopted daylight saving time and the British Army complied with it.
Divisional Routine Order 95
Tuesday, 13 June 1916
and many other examples
“British Armies in France will adopt the same time as ordered by recent decree of French Government, and at 11pm on 14th instant the time will be advanced 60 minutes and 11pm will become 12 midnight”.
This daylight saving lasted until 00am on 2 October when time was re-set to 11pm on 1 October. Throughout this period, time in use was at GMT plus one hour.
The cycles were similar in later years.
Adoption of 24 hour clock
The British Army adopted the use of the 24 hour clock (expressing time, for example as 23:59) at midnight 30 September – 1 October 1918.