Well, given the level that this has degenerated to,
I'm giving it to BFM_Lucky, as his guess was in the range.
Of course it would have to be a shadow clock, and in particular the first people to figure THAT out were the ones that built standing obelisks, and THAT happened to be around 3500 BC. (At least there is evidence of obelisks with timing marks around that time period.)
I've wondered a lot over the arbitrary number of time divisions being 12, and the best that I've ever been able to dig up as an answer is practical, but disappointing: That was as many lines as they could draw in before they started to run into each other making it hard/impossible to distinguish them apart!
