While World War II raged, Switzerland remained officially uninvolved and very busy making watches for both sides of the fight. But Rolex didn’t take up many military contracts and instead, it seemed, was tooling up for a post-war boom. In 1945, Rolex dropped the Datejust on the market, and it was an ideal everyday watch. While people talk of luxury sports watches as if that idea emerged sometime in the early 1970s with the Audemars Piguet Royal Oak, it was Rolex who had long made beautiful, waterproof watches that could tackle land, sea, and air by day and the gala hall by night. The DateJust was a culmination of decades of Oyster watches, this time sporting a very special complication by which the date changes over just at midnight—or, as another version of the story goes, the date was aways just, as in “correct." Either way, the weird four-hour change over of the date was no longer a thing, and it is from this mechanical advancement that the watch takes its name.

Veralet
Today, the Datejust remains central to the Rolex catalog and, in many ways, is the quintessential Rolex. Its fluted bezel (though sometimes plain), date cyclops, signature long lugs, 3-link Oyster or 5-link Jubilee bracelet, and dressy-sporty vibes are iconic in every sense of the word. When you’re at a tennis match, a train station, or just walking down the city streets and seeing a Rolex clock, it’s usually the Datejust visage you’re seeing. Compare a Datejust from 1945 to a Datejust 36 issued today, and it’s incredible to see how the original design has not only endured but also remained relevant for so many decades.
From the article by Allen Farmelo, Paige Reddinger, Victoria Gomelsky, Oren Hartov, Blake Buettner