Definition
A calendar month is one of the twelve named divisions of the year as they appear on the calendar — January, February, March, and so on — running from the first day of that month to its last day, without adjustment for the variable number of days each month contains. A calendar month may have 28, 29, 30, or 31 days depending on which month and whether it is a leap year.
The term operates in contrast to two other common measurements of monthly time:
1. Lunar month: A fixed period of approximately 28 days, tied to the cycle of the moon rather than the civil calendar.
2. Business or commercial month: An artificial construct of exactly 30 days, applied uniformly regardless of which calendar month it falls within. This is sometimes called an "arbitrary month" in older commercial usage.
When a statute, contract, or court rule specifies "calendar month" without further definition, it means the named month as it appears on the calendar — the full period from the first to the last day of that month, accepting its actual length.
Common Language
Wiktionary: "The period of duration from the first date of one month to the last date of the same month, which can thus vary from 28 to 31 days in length."
The common and legal meanings are largely consistent here, but a practical gap arises in legal computation. In ordinary speech, "a calendar month" simply describes one of the twelve months. In legal contexts — particularly in notice requirements, limitation periods, and contract deadlines — the question of when a calendar month begins and ends can carry significant consequences. A court computing a one-calendar-month deadline from March 15 does not simply ask "which month?" but must determine whether the period runs through April 14 or April 15. Ordinary usage does not demand this precision; legal usage requires it.
Common Confusion
Calendar month is frequently confused with:
- Month (unmodified): In many statutes and older instruments, "month" without qualification was historically interpreted as a lunar month of 28 days in English common law. Modern usage and most American jurisdictions have shifted to treating an unmodified "month" as a calendar month, but the historical ambiguity matters when interpreting older documents. Researchers should not assume that "month" and "calendar month" were synonymous in sources predating the twentieth century.
- Lunar month: 28 days, tied to the moon's cycle. Rare in modern legal drafting but appears in older English statutes and ecclesiastical contexts.
- Commercial or 30-day month: Used in financial instruments and interest calculations. A contract that says "30 days" is not the same as one that says "one calendar month," though the difference may be only one or two days in practice.
Why It Matters in Research
The central research trap is the historical instability of the word "month" itself. In English common law, "month" without a qualifier meant lunar month — 28 days. This rule persisted well into the nineteenth century and influenced early American legal materials. Researchers reading statutes, deeds, wills, or court opinions from the colonial period through the mid-1800s must be alert to this distinction. A limitation period or notice requirement expressed in "months" in an 1820 instrument may mean something numerically different from the same language in a 1920 statute.
Modern American statutes and rules almost universally define "month" as calendar month, either explicitly or through general definitions acts. But the corpus of historical legal sources does not share this uniformity.
A second trap is the mid-month computation problem. When a legal period runs for "one calendar month" from a date that is not the first of the month — say, from the 15th — courts have divided on whether the period ends on the 14th of the following month or the 15th. Some jurisdictions apply a corresponding-date rule; others measure to the end of the named month. This is not merely academic: it can determine whether a filing is timely or a notice effective.
Jurisdictional variations in computation statutes and rules are consequential. Researchers working across multiple jurisdictions should always check the applicable general definitions statute or court rule before relying on any assumed construction.
Historical Dictionary Support
Black's Law Dictionary supplies the core distinction directly: a calendar month is "one of the months of the year as enumerated in the calendar, January, February, March, etc., without reference to the number of days it may contain; as distinguished from a lunar month, of twenty-eight days, or a month for business purposes, which may contain thirty, at whatever part of the year it occurs."
This framing — calendar month versus lunar month versus commercial month — is the consistent spine of the legal dictionary tradition on this term. Black's is explicit that the distinguishing feature of the calendar month is its indifference to day-count: the month is what the calendar names it, not a fixed arithmetic unit.
What historical dictionaries do not address with useful specificity is the mid-month computation problem, or the transition in American practice away from the common law lunar month default. Researchers who find "month" in historical legal sources without a calendar qualifier should not rely on dictionary definitions alone; the applicable jurisdiction's interpretive rules and case law of the period will control.
Jurisdictional Note
Most American jurisdictions have enacted general definitions statutes that treat "month" as meaning "calendar month" unless context indicates otherwise. English common law's lunar month default is largely a historical artifact in American practice but remains relevant when interpreting older instruments or statutes enacted before a jurisdiction's definitions act took effect.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Time Computation in Law