Khan Academy
Khan Academy: Common Questions About Dates
Our calendar is called the Gregorian calendar and was instituted by Pope Gregory XIII in 1582. There are many other calendars. Learn about these calendars and the meanings of B.C.E and C.E.
Curated OER
Smithsonian Libraries: Scientific Identity: Pope Gregory Xiii (1502 1585)
A portrait of Pope Gregory XIII from the Dibner Library of the History of Science and Technology, made available through the Smithsonian Institution's Scientific Identity Collection.