There's a great new resource available to help keep the chronology of early Greek philosophers straight. Which thinkers were contemporaries, which ones were earlier and later. Previous references were all over the place so it's nice to have this all in one well-researched book.
Check out the publisher Cosmographia (cosmographia.net) and the main page for the book by Philip Thibodeau here:
I found out about it from the BMCR write-up here:
Enjoy! I sure am