Before the Datejust came along, the elements and technology to make it possible were already in place and in use by various watch companies. Calendar watches would often display the date around the dial’s periphery with a centrally mounted hand to indicate it. The window concept could also be found on such calendar watches where discs beneath the dial would display the month and day of the week.
Rolex’s contribution was simple and pragmatic, and it was intended more for the average consumer than the relatively complicated and expensive calendar watches were — even though the debut Datejust model itself was available only in 18k gold. The 1945 Rolex Datejust reference 4467 was 36mm wide and “waterproof” in the brand’s signature Oyster case, and the model was also used to introduce the now well-known Jubilee-style bracelet (the brand held a jubilee for its 40th anniversary to introduce the watch, and the name stuck)… More here.
You may already be aware of this evolution, but the article is still a good read nonetheless.