Definition
Bissextile refers to leap year — specifically, a year of 366 days in which an intercalary day is added to the month of February, making it a month of 29 rather than 28 days. The addition occurs every fourth year to reconcile the calendar year with the solar (tropical) year.
In legal contexts, the term carries a narrow but occasionally consequential meaning: it designates the extra day itself, or the year containing it, for purposes of computing time in contracts, statutes, limitations periods, and other instruments where the precise count of days matters.
Common Language
Modern common usage (Wiktionary): Having an intercalary day, particularly the quadrennial leap day of the Julian and Gregorian calendars traditionally placed as a "second sixth" day before March 1st.
Historical common usage (Webster's 1913): Leap year; every fourth year, in which a day is added to the month of February on account of the excess of the tropical year (365 d. 5 h. 48 m. 46 s.) above 365 days.
The common and legal meanings are largely identical in substance — both mean leap year or the leap day. The legal dimension adds one layer of practical specificity: courts and statutes have had to decide whether the added day counts as a distinct legal day or merges with an adjacent day for purposes of time computation. That question, not the calendar mechanics, is where legal meaning diverges from common usage.
Common Confusion
Bissextile is sometimes loosely used to mean the leap day only (February 29) rather than the full leap year, and historical sources use the term both ways. Researchers should note which sense a source intends. Additionally, because the Roman calendar placed the intercalary day on the sixth day before the Kalends of March — meaning it fell on what we now call February 24 — early statutes and commentaries occasionally treat February 24 and 25 as the doubled days, not February 28 and 29. This matters when reading pre-modern instruments that specify dates in February of a leap year.
Why It Matters in Research
The practical legal significance of bissextile is narrow but real in time computation. The central question historical courts faced: when a statute of limitations, contract deadline, or other period is measured in years or days, does the leap day count as an additional day, extending the period, or does it collapse with February 28 into a single legal day?
The English statute 21 Henry III resolved this by providing that February 28 and February 29 together count as one day in leap year — meaning a period measured in days does not automatically gain a day simply because February has 29 entries on the calendar that year. Bouvier notes this statute was received in some American jurisdictions, citing Indiana and Pennsylvania authority. Researchers working in those jurisdictions on pre-code time computation questions should check whether the Henry III rule was treated as part of the received common law.
For corpus researchers, bissextile appears rarely in American case law and almost never in statutory text after the nineteenth century. When it does appear, it is almost always in the context of computing the duration of a lease, a limitations period, or an age calculation. More modern instruments simply say "leap year" or rely on standard time-computation rules that treat February 29 as an ordinary calendar day.
The deeper trap for historical research is the Roman calendar origin: the bissextile day was interpolated as a repetition of the sixth day before the Kalends of March (sexto calendas), making February 24 the "doubled" day in classical and early medieval reckoning. English law eventually settled on February 29 as the intercalary day, but early English and Continental materials may date events inconsistently as a result.
Researchers should also be alert to the century-year exception: years divisible by 100 are not leap years unless also divisible by 400 (e.g., 1900 was not a bissextile year; 2000 was). This distinction, invisible in most legal instruments, can affect day-count calculations in long-term instruments such as bonds, leases, or pension agreements spanning a century boundary.
Historical Dictionary Support
The five source dictionaries present a consistent and complementary picture, with varying levels of depth.
Black's (1st ed.) is fragmentary as reproduced, offering only the opening clause. Black's (2nd ed.) supplies the working definition: the day added every fourth year to February to align calendar and solar year, resulting in a year of 366 days. Anderson's merely cross-references YEAR without elaboration.
Bouvier adds the most legally operative content, noting the Henry III statute and its American reception, and preserving the Roman etymology connecting the term to the doubled sixth day before the Kalends of March. Burrill provides the most complete etymological and calendrical explanation: bissextile derives from bis (twice) and sextilis (sixth), because the sixth day before the Kalends of March was counted twice — specifically, February 24 was reckoned twice in every fourth year. Burrill cites Cowell, confirming the term had sufficient legal currency to warrant treatment in earlier English legal lexicography.
What the historical dictionaries collectively miss is any sustained treatment of how American courts applied or modified the Henry III rule, or guidance on the century-year exception introduced by the Gregorian calendar reform of 1582 — a reform England did not adopt until 1752, meaning early American colonial instruments may have been drafted under the Julian calendar convention.
Jurisdictional Note
The English statute 21 Henry III, providing that February 28 and 29 in leap year count as one legal day, was received as part of the common law in some American states. Indiana and Pennsylvania are specifically noted in Bouvier's authority. Researchers working on historical time-computation questions in other jurisdictions should not assume reception without verification; most modern American jurisdictions treat February 29 as an ordinary calendar day by default under general time-computation statutes or rules of court.