Legal Definitions - de anno bissextili

De anno bissextili is a historical legal principle originating from 13th-century England. It is Law Latin for "of the bissextile year," referring to a leap year. This principle was a directive issued by King Henry III to judges, advising them on how to calculate legal deadlines that were set for "within a year" when that year included a leap day.

Specifically, the rule stated that when determining a one-year period that encompassed February 29th of a leap year, both February 28th and February 29th were to be counted as a single day. The purpose was to ensure that a "year" for legal purposes consistently represented 365 days, preventing disputes or confusion that might arise from the extra day in a leap year.

Here are some examples illustrating how this historical rule would have applied:

• Feudal Service Obligation: Imagine a vassal in 1252, which was a leap year, who received a land grant from their lord on March 1st. The grant stipulated that the vassal must perform a specific military service "within one year" of receiving the land. The standard deadline would be March 1st, 1253. According to the *de anno bissextili* rule, when calculating this one-year period, February 28th and February 29th of 1252 would be treated as a single day. This ensured that the vassal's obligation was due after a consistent 365 days, preventing any argument that the leap year either extended or shortened their duty period.

• Legal Claim Deadline: Consider a plaintiff in 1256, another leap year, who suffered an injury on April 1st and had "one year" from that date to file a legal claim for damages. The natural deadline would be April 1st, 1257. Under the *de anno bissextili* principle, the calculation of this one-year period would count February 28th and February 29th of 1256 as a single day. This meant the plaintiff's legal window to bring their claim was precisely 365 days, providing clarity and consistency in legal timelines despite the occurrence of a leap year.

• Apprenticeship Term: Suppose a young person in 1260, a leap year, began an apprenticeship on May 1st with a contract specifying a term of "one year." The expected completion date would be May 1st, 1261. To determine the exact end of the apprenticeship, the *de anno bissextili* rule would apply, treating February 28th and February 29th of 1260 as one day. This ensured that the apprenticeship legally lasted for 365 days, preventing either the master from demanding an extra day of service or the apprentice from claiming early completion due to the leap year's additional day.