Pentecontad calendar
The pentecontad calendar (from Koine Greek: πεντηκοστή, romanized: pentēkostē, lit. 'fiftieth') is an agricultural calendar system thought to be of Amorite origin in which the year is broken down into seven periods of fifty days (a total of 350 days), with an annual supplement of fifteen or sixteen days. Identified and reconstructed by Julius and Hildegaard Lewy in the 1940s, the calendar's use dates back to at least the 3rd millennium BCE in western Mesopotamia and surrounding areas.