Neugebauer was also interested in chronology. He was able to reconstruct the Alexandrian Christian calendar and its origin from the Alexandrian Jewish calendar as of about the 4th century, at least 200 years prior to any other source for either calendar. Thus, the Jewish calendar was derived by combining the 19-year cycle using the Alexandrian year with the seven-day week, and was then slightly modified by the Christians to prevent Easter from ever coinciding with Passover. The ecclesiastical calendar, considered by church historians to be highly scientific and deeply complex, turned out to be quite simple.