Which is, in fact, GMT. In 1918, France was in the same time zone as Great Britain, GMT (or what would nowadays be called UTC). It had switched its legal time from Paris mean time (i.e., the solar time on the Paris meridian, which is ahead of GMT by about nine minutes) to GMT in 1911 and did not switch to Central European Time (GMT/UTC plus one hour) until 1940, which was imposed by the German occupation authorities and never reversed after WWII.