onemonth

Onemonth refers to a unit of time that can mean either a calendar month or a fixed 30-day interval, depending on the context. In the Gregorian calendar, calendar months vary in length, with 28, 29, 30, or 31 days. The calendar-month duration is typically defined as the interval from a given date to the same day in the following month, with adjustments when the following month has fewer days.

The word month has historical roots linked to the Moon’s cycle, and most month names in the Gregorian calendar derive from Latin, Roman gods, or historical figures. January is named for Janus, February for februa, and July and August for Julius Caesar and Augustus, among others. The general concept of a month reflects both astronomical and cultural timekeeping practices that organized the year into recurring periods.

Usage and applications vary by field. In finance, subscriptions, payroll, and project planning, one month often denotes a typical monthly period aligned with calendar months, though some calculations treat a month as a fixed 30-day block for simplicity. In computing, date arithmetic distinguishes calendar months from fixed-day intervals, which can affect due dates, interest accrual, and scheduling. In ISO 8601, a period of one calendar month is encoded as P1M, a duration that corresponds to the length of the month in question rather than a fixed number of days.

Notes: a month is not a fixed duration and can differ by month and calculation method. See also calendar month, Gregorian calendar, and ISO 8601.