Definition
A unit of time measurement with multiple legal meanings depending on context, jurisdiction, and the type of month intended. When a legal instrument, statute, or rule uses the word "month" without qualification, determining which of the following meanings applies is a threshold research question.
1. Calendar month (civil or solar month): A month as it appears on the Gregorian calendar — January, February, March, and so forth — running from the first to the last day of that month, or from a given date in one month to the corresponding date in the following month. This is the dominant modern legal meaning in American law and in most statutory contexts.
2. Lunar month: A period of twenty-eight days, or four weeks. In historical English common law, "month" in temporal matters meant a lunar month, not a calendar month. This rule was well-established and the distinction was not trivial — a lunar month is always twenty-eight days, while calendar months range from twenty-eight to thirty-one.
3. Astronomical month: One-twelfth of the solar year, approximately 30.44 days. Relevant in technical or scientific contexts; legal instruments are not understood to invoke this meaning.
4. Month of grace (historical): In old English forest law, a period occurring in the middle of summer — roughly June 15 to July 15 — during which hunting deer in the royal forest was unlawful, that being the fawning season. The deer were said to be "defended" during this period, giving rise to the name. This meaning is obsolete and appears only in historical English forest law sources.
Common Language
Modern common usage (Wiktionary): A period into which a year is divided, historically based on the phases of the moon; informally, a period of thirty days or thirty-one days depending on the calendar.
Historical common usage (Webster's 1913): One of the twelve portions into which the year is divided, corresponding nearly to the length of a synodic revolution of the moon. In popular use, a period of four weeks is often called a month.
Editorial note: The common language gap here is historically significant. Ordinary speakers — then and now — use "month" to mean a calendar month. But the English common law rule was the opposite: "month" in a legal instrument meant a lunar month of twenty-eight days unless the instrument specified otherwise. American law gradually reversed this default. A researcher encountering an eighteenth- or nineteenth-century document must not assume "month" means what a modern reader expects.
Common Confusion
The lunar/calendar distinction has generated persistent confusion in legal texts. A promise to perform within "one month" meant different things depending on whether English common law or a particular American jurisdiction's rule applied — and the difference could be two to three days. Courts and commentators in the nineteenth century frequently noted this ambiguity. Burrill flags the English default explicitly; Bouvier notes that American practice diverged. Researchers working across jurisdictions or eras should never assume uniformity.
Why It Matters in Research
The word "month" is a hidden variable in historical legal documents. Its meaning shifted over time and was never uniform across jurisdictions. Key traps:
Temporal direction: English common law treated "month" as twenty-eight days in temporal matters. Many early American courts followed this rule; others rejected it in favor of calendar months. A limitations period, notice requirement, or contractual deadline that reads "one month" in an 1830 American document may have been interpreted differently in New York than in Virginia, and differently again than an equivalent English document from the same period.
Statutes versus instruments: American legislatures frequently defined "month" by statute, often specifying calendar month. Where a statute controls, the common law default is displaced. When no statute applies, courts look to the instrument's context and jurisdiction.
Overlap with "moon": In older English legal literature, "moon" and "month" were sometimes used interchangeably in colloquial legal writing, reflecting the term's lunar origins. This is not standard legal usage but appears in informal texts and some early treatises.
Forest law meaning: The "month" of forest law (the fawning close season) appears in Manwood's treatise on forest law and in Cowell's legal dictionary. It will surface in corpus searches on English forest law, deer, game law, and royal forests. It has no application outside that narrow historical context.
Corpus navigation: Searches on limitations periods, notice requirements, contract performance deadlines, and procedural time rules will frequently surface "month" as a critical term. Filter by jurisdiction and date. Post-codification sources are more likely to define "month" explicitly; pre-codification sources require inference from context and applicable common law rule.
Historical Dictionary Support
The historical dictionaries converge on the core distinction but vary in emphasis.
Burrill is the most precise: he identifies the lunar month (twenty-eight days, four weeks) as the English common law default for temporal matters, cites Coke on Littleton and Blackstone's Commentaries, and signals that the rule was a matter of settled common law, not mere convention.
Bouvier gives the fullest typology — astronomical, civil/solar, and lunar — and states clearly that when "a month simply is mentioned" in law, the astronomical meaning is excluded. He addresses the civil month as the Gregorian calendar standard and the lunar month as the historical default, providing useful context for American practice in transition.
Black's (both editions) notes the calendar month as the general modern meaning and preserves the forest law meaning separately. The entry is compressed and does not fully address the common law/civil law divergence that Bouvier and Burrill handle.
Rapalje & Lawrence and Anderson's Dictionary of Law do not offer substantive entries on "month" in the source material provided — the excerpts appear to be misaligned or fragmentary. Researchers should not rely on those sources for this term.
What the historical dictionaries collectively miss: the jurisdictional variation within American states. No single historical dictionary maps which American jurisdictions had, by statute or case law, adopted the calendar month as default by a given date. That research requires jurisdiction-specific statutory and case law review.
Jurisdictional Note
Most American jurisdictions have resolved the lunar/calendar ambiguity by statute, typically in favor of the calendar month. Federal law and the majority of state codes now define "month" as a calendar month when used in a statute. However, contract interpretation may still look to common law defaults where a contract fails to specify, and older instruments predating codification may require historical analysis of the applicable jurisdiction's rule.
Encyclopedia Cross-Reference
Law Mind Encyclopedia — Time Computation
Law Mind Encyclopedia — Statutes of Limitation
Law Mind Encyclopedia — Contract Performance and Conditions