Definition
Dies excrescens (Latin: "the increasing day") is the additional day added to the calendar in a leap year — February 29. In medieval English law, this extra day had procedural significance: courts and legal instruments operated on annual cycles, and the appearance of an unexpected day created questions about how it should be counted in the computation of time, the running of legal terms, the performance of obligations, and the duration of estates or leases measured in years.
Why It Matters in Research
The term appears almost exclusively in medieval and early common law sources. Researchers working in pre-modern English legal records — writs, plea rolls, property conveyances, or treatises on seisin and tenure — may encounter dies excrescens in discussions of time computation. The core legal question was not exotic: when a lease, obligation, or legal term was set to run for a fixed number of years, what happened to the surplus day in a leap year? Did it belong to the tenant or the landlord? Was it counted or ignored?
The practical stakes were real in an era when annual rents, terms of court, and possessory rights were calibrated strictly to the year. Researchers should not treat dies excrescens as mere calendrical trivia. In the context of a property dispute or a plea in abatement, the character of February 29 could affect whether a writ was timely, a term had expired, or a condition had been performed.
The term will not appear in modern legal sources. It belongs to the era of Bracton and the Year Books. If a research path leads here, the next step is almost certainly into the Bracton text itself (De Legibus et Consuetudinibus Angliae, fols. 359–359b) or into secondary scholarship on medieval English time-reckoning and legal procedure. Both Black's and Burrill's cite Bracton as the authoritative source; no subsequent English or American case law is expected to turn on this concept.
Researchers in legal history examining the computation of time across different calendrical systems — Julian versus Gregorian transitions, the shift of the English new year, or the treatment of fractions of time in equity — may find dies excrescens a useful reference point for how the common law handled calendrical anomalies before modern statutory rules on time computation displaced the old learning entirely.
Historical Dictionary Support
Black's Law Dictionary and Burrill's Law Dictionary agree precisely, both citing Bracton fols. 359 and 359b as the source authority. Neither dictionary elaborates beyond the bare definition. This convergence reflects the term's narrow scope: it is a term of art with a single, stable meaning that never expanded beyond its medieval context.
Both entries use the phrase "old English law," signaling to the reader that this is a historical artifact rather than a live legal concept. The brevity of both treatments is itself instructive — neither Bouvier, nor later American dictionaries, nor equity treatises appear to develop the concept further. What the historical dictionaries do not address is the substantive legal treatment: Bracton's discussion at the cited folios is the place to look for how the day was actually handled, not the dictionary entries themselves.
Jurisdictional Note
Dies excrescens is a concept of medieval English common law and has no recognized place in modern American, English, or Commonwealth legal practice. Statutory rules governing the computation of time now govern all jurisdictions, typically treating a year as 365 days or specifying leap year treatment explicitly.