This claim originally comes from Diogenes Laërtius, a biographer of ancient Greek philosophers, who wrote (Lives of the Philosophers (Philosophoi Biol), book I # 27):
[Thales] is said to have discovered the seasons of the year and divided it into 365 days
(You can read the entire bio here.)
A number of people besides Kenny repeat this assertion uncritically.
Thales of Miletus lived circa 620 BCE to circa 546 BCE. Diogenes lived circa 180 CE - circa 240 CE, quite a bit later.
However I find it hard to believe that the Egyptiians were able to predict when the Nile would flood thousands of years before Thales without realizing that there are approximately 365 days in a year.
According to his page in the Internet Encyclopedia of Philosophy
Thales’s Discovery of the Seasons
From Diogenes Laërtius we have the report: ‘[Thales] is said to have discovered the seasons of the year and divided it into 365 days’ (D.L. I.27). Because Thales had determined the solstices, he would have known of the number of days between say, summer solstices, and therefore have known the length of a solar year. It is consistent with his determination of the solstices that he should be credited with discovering that 365 days comprise a year. It is also a fact that had long been known to the Egyptians who set their year by the more reliable indicator of the annual rising of the star Sirius in July. Thales may have first gained the knowledge of the length of the year from the Egyptians, and perhaps have attempted to clarify the matter by using a different procedure. Thales certainly did not ‘discover’ the seasons, but he may have identified the relationship between the solstices, the changing position during the year of the sun in the sky, and associated this with seasonal climatic changes.
According to the book Thales of Miletus: The Beginnings of Western Science and Philosophy by Patricia F. O'Grady,
If [Thales] advocated the development of a calendar based on the 365 days of the year, there is nothing to testify to the fact apart from the report of Digenes.