The original “Zenith” Daytona housed Zenith’s Cal. 4030 movement. Rolex heavily modified what was in fact the El primero engine, and beside other modifications altered the beats per hour count from 36,000 to 28,800 (from 5 to 4 Hertz). In fact, only some 50% of the parts of the original Zenith movement were untouched after the modification.
Notwithstanding the success of the 4030, Rolex had been working on the new in-house movement for some time before it’s launch at the Basle World Fair in 2000. On October 31st 1995, a patent with drawings of parts of the movement construction was applied for (Swiss patent number CH199 5000003077), and on October 29th 1996 Rolex applied for further patents of the movement (US patent number 5.793.708). This was, of course, what transpired to be the in-house cal. 4130 – the movement rated as one of the top ten ever made.
Aside from this, of course, the Daytona is – quite simply – a beautiful watch.