The first dual-time wristwatch (1925)

The Longines of 1925. Beside the ordinary hour and minute hands sits a third, orange-tipped hour hand for the second zone, read on the same twelve-hour dial. The small figures inside the hour numerals, 13 to 24, give the afternoon hours in twenty-four-hour form; the seconds have their own small dial at six. Under the name is a signal flag for the letter Z.Photograph: Longines

The need: Ships and, increasingly, aircraft kept their logs and radio schedules in Greenwich time, while the people on board lived by the clock of wherever they were. A navigator needed both times on the wrist without adding and subtracting.
Made by: Longines, Saint-Imier. Longines describes it as the first dual-time wristwatch, showing local time and Greenwich Mean Time on one dial.
What was new: A second hour hand on the same centre as the first, so that one twelve-hour dial carries two times at once: local time on the ordinary hands, the other zone on the coloured hand. It is the layout that dual-time watches still use.
Worth knowing: This is a dual-time watch, not a GMT. The second time is read in twelve-hour form, so the dial alone cannot say whether it is morning or evening in the other zone. The 24-hour hand and bezel that solve that came later; see GMT and dual time.
Where it is now: The photograph comes from Longines’ own heritage material. We have not found the model in a public sale.

Historian’s noteLongines sells the modern versions as “Zulu Time” and links the name to the Z flag on the dial, the letter that stands for Greenwich time. The spoken word “Zulu” for Z comes from the phonetic alphabet adopted internationally in the 1950s, so the name is best read as a modern one. The claim to be the first dual-time wristwatch is Longines’ own; we have found nothing earlier, but nothing independent either. Longines had already made dual-time pocket watches, its Turkish watches of 1908.

SourcesLongines, What is a GMT watch