More on Gregorian Epoch from Infoplease:

• Gregorian Epoch - Gregorian Epoch The epoch or day on which the Gregorian calendar commenced—March, 1582. ...
• Saint Gregory VII: Bibliography - Bibliography See his Correspondence (tr. 1932, repr. 1969); S. Williams, ed., The Gregorian Epoch ...
• Dictionary of Phrase and Fable: G - Definitions, origins, and illustrative excerpts for words, phases, and literary allusions starting with "G"