"Let's synchronize our watches!"
When did that question begin to make sense? In a fascinating essay from Geographical Imaginations, "Homogeneous (war) time," we have the answer: The Western Front in World War I:
‘When watches were synchronised what, exactly, were they synchronised to, and how was it done?’
Two technologies were pressed into service by the Allies; they can both be seen in this synchronisation instruction contained in Operation Order (no 233) from the 112th Infantry Brigade on 10 October 1918:
O.C. No.2 Section, 41st Divisional Signal Company, will arrange for EIFFEL TOWER Time to be taken at 11.49 on “J” minus one day ["J" was the day of the attack] and afterwards will synchronise watches throughout the Brigade Group by a “rated” watch.
The first was the Eiffel Tower – or, more accurately, the time-signal transmitted from the Eiffel Tower throughout the war. In 1909 the original twenty-year lease for the Tower was about to expire, and many Parisians loathed it (Maupassant famously had lunch there every day because it was the one place in the city from which it couldn’t be seen) so that its demolition seemed imminent. But it was saved in large measure because the French military was persuaded of its strategic value as a navigation and wireless beacon. Eiffel had allowed the Minister of War to place antennas at the top in 1903, and the Bureau des Longitudes (under the direction of Henri Poincaré) urged the development of the military radio-telegraphic station to broadcast time-signals twice daily. The original intention was to enable mariners to set their chronometers, but the project had a wider strategic, scientific and symbolic significance. ‘Wireless simultaneity’, writes Peter Galison, ‘had become a military as well as a civilian priority’. ....
The second requirement for choreographing time in the battlespace was the wristwatch. Originally wristwatches were designed for women (the first “wristlet” was made by Philippe Patek in 1868), and although the Kaiser had 2,000 wristwatches made for his naval officers in 1880 – and there is some evidence of their use in the Boer War – men continued to favour pocket-watches until the First World War. Both soldiers and aviators needed a hands-free way of telling the time, and so the “trench watch” was born. In Knowledge for War: Every officer’s handbook for the front, published in 1916, a wristwatch headed the kit list, above even a revolver and field glasses, and in the same year one manufacturer claimed that ‘one soldier in every four’ was already wearing a wristwatch ‘and the other three mean to get one as soon as they can.’
I'm sure there's a lot else to be said on this topic, including the role of the railroads, their schedules, "time zones," and the pocket watch from which the wrist watch evolved. (My father's gold pocket watch financed my first trip to Thailand, in a rather neat metaphor.)
Of course, today, we have cell phones, pads, and laptop computers, all synchronized automagically from NTP (network time protocol) servers and ultimately from "atomic clocks." Why, it's almost as if we're living in a war zone!