Richards, E. G. (2012). "Calendars"(PDF). In Urban, Sean E.; Seidelmann, P. Kenneth (eds.). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 606. ISBN978-1-891389-85-6. Archived from the original(PDF) on 30 April 2019.
Richards, E. G. (2012). "Calendars"(PDF). In Urban, Sean E.; Seidelmann, P. Kenneth (eds.). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 606. ISBN978-1-891389-85-6. Archived from the original(PDF) on 30 April 2019.