Dates

Basically, dates are approximate. The dates assigned to most of Leonardo's works (whether art, architecture, engineering, anatomy etc.) vary in sources, often by several years, so the dates I give are those which I personally trust more depending which source or which I believe are more plausible given my own amateur comparison of evidence. In many cases there is inaccuracy or even just approximation by those who reported events whether contemporaries of Leonardo or later writers.

The length of time assigned by various sources is also accounted for by the fact that Leonardo would have worked, often for years, on his designs. Apart from certain presentational drawings, the nature of Leonardo's drawings tends to be that of work-in-progress drawings, often with some details not fully drawn.

In addition, the chronological reckoning differed in Leonardo's time. The Maestro writes in the introduction to his notebooks: "Begun at Florence, in the house of Piero di Braccio Martelli, on the 22nd day of March 1508. And this is to be a collection without order, taken from many papers which I have copied here, hoping to arrange them later each in its place, according to the subjects of which they may treat. But I believe that before I am at the end of this [task] I shall have to repeat the same things several times; for which, O reader! do not blame me, for the subjects are many and memory cannot retain them [all] and say: 'I will not write this because I wrote it before.' And if I wished to avoid falling into this fault, it would be necessary in every case when I wanted to copy [a passage] that, not to repeat myself, I should read over all that had gone before; and all the more since the intervals are long between one time of writing and the next.

Notes:

1. The Christian era was computed in Florence at that time from the Incarnation, namely Lady Day on 25th March. Hence this should be 1509 by our reckoning. Incidentally, in the history of Florence in the early part of the 16th century Piero di Braccio Martelli is often mentioned as Commissario della Signoria. He was famous for his learning and at his death left four books on Mathematics ready for the press.
2. The Julian Calendar was not replaced by Pope Gregory XIII with the Gregorian one until October 1582. Not all sources convert the dates, and none of the sources I consulted said whether or not they have done so.

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