Definition
Bissextus (also spelled bisextus) is the intercalary day added to the calendar every four years to reconcile the civil year with the solar year — in modern terms, February 29th of a leap year. In the Roman calendar from which the legal usage derives, this day was created by counting twice the sixth day before the Calends of March (ante diem sextum Kalendas Martias), giving rise to the name: bis (twice) + sextus (sixth). English law inherited both the term and the underlying computational problem it solved, and the day appears in legal texts under the alternative form bisextus.
The legal significance of bissextus arises not from the day itself but from the question of how it is to be counted — or whether it is to be counted at all — in the computation of legal time periods. When a statute, instrument, or court rule requires that an act be performed within a fixed number of days, does the intercalary day add one day to the available period, or is it absorbed into the year without independent legal weight? The old English answer, carried forward from Roman practice, treated the bissextile day and the day preceding it as a single legal day, meaning the leap year was not counted as having 366 days for purposes of annual computation. Modern courts and procedural rules generally count February 29 as any other calendar day when computing day-by-day periods, but the issue could arise under instruments or statutes that speak in years rather than days.
Common Language
Modern common usage (Wiktionary): Bissextus is defined as the extra day added in a leap year, February 29; also used adjectivally in "bissextile year" to mean a leap year.
Historical common usage (Webster's 1913): Webster's defines bissextile as relating to the leap year, from bissextus, the intercalated day. It treats the term as substantially equivalent to leap year or leap day with no technical legal valence.
The gap is narrow but real. In ordinary English, bissextus is simply a slightly formal name for February 29, with no operational significance. In legal usage, the term carries a specific computational rule — the Roman and early English convention that the bissextus and the preceding day together constitute one dies in law — a rule that could affect whether deadlines, terms of years, or periods of limitation run differently in leap years than in other years.
Why It Matters in Research
This term is rarely encountered outside of old English legal texts, civil law commentaries, and treatises on the computation of time. Researchers are most likely to meet it in three contexts.
First, in historical sources dealing with the calendar. Pre-modern legal documents sometimes note that a transaction fell on the bissextus or describe a period running across it. Understanding the term is necessary to parse the date correctly and to know whether the writer intended the day to carry independent legal weight.
Second, in treatises and digests addressing the law of time computation. Burrill's entry connects the bissextus to the broader doctrine governing how fractional days and added days are handled in legal periods. Researchers working on statutes of limitation, prescription, or contractual notice periods in civil law or mixed jurisdictions may find the historical treatment of bissextus cited as a precedent for how to handle the leap day.
Third, in canon law and ecclesiastical records, where the Roman calendar survived longest in administrative practice and where the bissextus appears with some regularity in older instruments.
Be alert to spelling variation. Burrill cross-references bisextus and bissextus as equivalent. Latin dictionaries treat the single-s form as a variant. Both forms appear in legal sources; searching only one may miss relevant material.
Historical Dictionary Support
Burrill's Law Dictionary provides the primary entry. Burrill defines bissextus as the "added, repeated or increasing day (dies excrescens) of a leap year," describing it as the result of accumulated fractional hours from the three preceding years combined with those of the fourth. He traces the term to the Roman calendar's practice of counting the sixth day before the Calends of March twice, and cites Brissonius (Barnabé Brisson's De Verbis Juris) as the supporting civil law authority.
The entry is instructive for what it emphasizes: the dies excrescens framing — the day as a day that "grows out of" accumulated surplus — reflects the Roman and civilian view that the bissextus was not an ordinary day but a corrective insertion, which supported the doctrine that it shared its legal identity with the day before it. This is the doctrinal weight behind the term that modern general dictionaries miss entirely.
Burrill's entry is brief and does not address how English common law courts actually resolved leap-year computation disputes or whether the Roman rule was received wholesale. Researchers should treat the entry as an introduction to the vocabulary and the civil law background rather than a complete account of English practice.
Jurisdictional Note
The Roman rule treating the bissextus as legally merged with the preceding day was a feature of civil law systems and ecclesiastical practice. English common law courts did not uniformly adopt it, and the question of how February 29 is counted in a leap year under any particular instrument or statute remains one of construction rather than settled universal rule. Modern procedural codes in most common law jurisdictions count all calendar days without exception unless the rule expressly provides otherwise.