E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Annus magnus. The Chaldaic astronomers observed that the fixed stars shift their places at about the rate of a degree in seventy-two years, according to which calculation they will perform one revolution in 25,920 years, at the end of which time they will return to their as you were. This revolution of the fixed stars is the annus magnus. The Egyptians made it 30,000 years, and the Arabians 49,000. (See Abulhasans Meadows of Gold.)
¶ For a year and a day. In law many acts are determined by this period of timee.g. if a person wounded does not die within a year and a day, the offender is not guilty of murder; if an owner does not claim an estray within the same length of time, it belongs to the lord of the manor; a year and a day is given to prosecute appeals, etc.