The Pentecontad Calendar is a unique agricultural calendar system thought to be of Amorite origin in which the year is broken down into seven periods of 50 days, with an annual supplement of fifteen or sixteen days. Identified and reconstructed by Hildegaard and Julius Lewy in the 1940s, the calendar's use dates back to at least the 3rd millennium BCE in Western Mesopotamia and surrounding areas. Used well into the modern age, forms of it have been found in Nestorianism and among the fellaheen of modern Palestine.