Euler's preface to the work "An examination of the Middle Ages, set forth according to astronomical and chronological principles," by Heinrich Wilhelm Clemm, Berlin 1752
Euler considers the change of the length of months and years since ancient times.
Original Source Citation
Berlin: Ambrosius Haude and Johann Carl Spener, pp. 2018-05-10.
Opera Omnia Citation
Series 2, Volume 30, pp.151-152.