Definition
Bisextus (also spelled bissextus) is the Latin term for the intercalary day inserted into the Roman calendar every four years to reconcile the civil year with the solar year. In legal contexts, the term refers to the leap day — the extra day added to February in a leap year — and carries significance for computing legal time periods, deadlines, and the calculation of dates in instruments, statutes, and court proceedings.
The Roman calendar originally designated this day as a repetition of the sixth day before the Kalends of March (ante diem sextum Kalendas Martias), hence the name: bis (twice) + sextus (sixth). In practical legal usage, however, the term functions simply as the name for February 29th and the legal questions that day generates.
Why It Matters in Research
The primary research interest in bisextus is its role in time-computation disputes. When a statute, contract, or court rule prescribes a period measured in years, the question arises whether a leap year's extra day extends or collapses a deadline. Early common law sources treated the leap day as legally merged with February 28th — the two days were counted as one for purposes of annual reckonings — a rule with direct consequences for statutes of limitations, terms of years in property conveyances, and annuity calculations.
Researchers working in pre-modern legal materials should be alert to the fact that the treatment of the leap day was not uniform across jurisdictions or time periods. A contract term expressed in years might be computed differently depending on whether the governing court recognized the leap day as a distinct unit or absorbed it. This distinction matters most in:
— Statutes of limitations running across a leap year boundary
— Lease terms and tenancies measured in years
— Age calculations (coming of age, majority) falling in a leap year
— Ecclesiastical and probate instruments dated by the Roman calendar system
The Digest reference cited in Burrill (Dig. 50.16.98) is a genuine Roman law source dealing with the legal definition of a year and the treatment of the intercalary day under Roman civil law. Researchers consulting civilian or mixed-jurisdiction sources will find this the foundational text. The passage addresses how the annus (year) is to be computed for legal purposes, and the intercalary day's ambiguous status within that reckoning is squarely at issue.
Modern legal research rarely requires engaging bisextus as a technical term — contemporary courts and statutes address leap-year computation directly — but historical research into instruments, wills, and litigation from the Roman, medieval, or early modern periods may require understanding how this day was classified and counted.
Historical Dictionary Support
Burrill's entry is a pass-through, directing the reader to "Bissextus" (the alternate spelling) without elaborating. The Digest citation (Dig. 50.16.98) is the substantive source Burrill points toward. That passage, from the title De verborum significatione in the Digest of Justinian, addresses definitional questions about time units in Roman law. The intercalary day appears there in the context of whether a full year had elapsed for legal purposes when the period included the extra February day.
The alternate spelling bissextus is the more common form in classical and legal Latin sources; bisextus with a single 's' appears as a variant. Researchers should search both forms in historical databases and manuscript sources. Historical legal dictionaries treat the two spellings interchangeably.
What historical dictionary sources do not adequately address is the downstream common law treatment. The Roman civil law rule — that the intercalary day was merged with the day preceding it — was received into English legal practice in modified form, but the precise contours of that reception are not well synthesized in the major historical legal dictionaries. Researchers needing that analysis will need to consult treatises on legal computation of time rather than dictionary sources alone.
Jurisdictional Note
The Roman law treatment of bisextus directly governs in civilian jurisdictions tracing their rules on time computation to the Corpus Juris Civilis. Common law jurisdictions developed analogous but not identical rules. Modern statutory and court-rule frameworks in most jurisdictions now explicitly address leap-year days in time-computation provisions, largely displacing the historical common law ambiguity.