Month (mo)
The month is one of the most practically useful but mathematically imprecise units of time. In everyday life, a month means one of the 12 named calendar periods of the Gregorian year, ranging from 28 days (February in a common year) to 31 days (January, March, May, July, August, October, December). For conversion purposes, this converter uses the average month of the Gregorian calendar: the Gregorian year of exactly 365.2425 days divided by 12, giving 30.436875 days or exactly 2,629,746 seconds. This is the most defensible average, used by most scientific and engineering conventions, and it is close enough for all practical purposes while being transparently defined.
Definition
For this converter, one month equals 2,629,800 seconds, or approximately 30.4375 days, derived from the Gregorian average year (365.2425 days) divided by 12. Calendar months in the Gregorian system vary from 28 days (February in common years) to 29 days (February in leap years) to 30 days (April, June, September, November) to 31 days (January, March, May, July, August, October, December).
History
The month is one of humanity's oldest time units, derived from the cycle of the Moon. The word month shares its root with Moon in English, as do their equivalents in many other languages. Early lunar calendars divided the year into months based on the synodic month, the time from one new Moon to the next, approximately 29.53 days. Twelve such months give 354 days, about 11 days shorter than the solar year, which causes purely lunar calendars to drift through the seasons. The Islamic Hijri calendar remains a pure lunar calendar today, which is why Ramadan moves through all seasons over a 33-year cycle. Most ancient cultures eventually adopted lunisolar calendars that inserted extra months periodically to keep the calendar aligned with the seasons. The Gregorian calendar, introduced in 1582, uses months of fixed length that have no direct relationship to the lunar cycle, with the occasional leap day inserted to track the solar year.
Common Uses
Months are the dominant unit for medium-term financial and contractual planning. Mortgages, loans, and leases are expressed in monthly payments. Subscription services charge monthly fees. Employment contracts specify monthly salaries. In medicine, pregnancy is tracked in both weeks and months, with a full-term pregnancy lasting about 9 months or 40 weeks. Drug treatment regimens are often described in months: a 6-month course of tuberculosis treatment, a 12-month follow-up period. In business, monthly reporting cycles drive accounting, budgeting, and performance review processes.
Did You Know? Facts About Month
- The uneven lengths of calendar months trace back to Roman calendar politics. The original Roman calendar had months of varying length, and when Julius Caesar reformed it in 46 BCE, the lengths were rationalized but unevenly distributed. When Augustus Caesar named a month after himself (August), he reportedly extended it to 31 days to match Julius Caesar's July, taking a day from February — which is why February is so short.
- Twelve months of 30 days each would give a 360-day year, which is why many ancient cultures used 360 as a base for geometry and angle measurement. The 360-degree circle is a relic of the 12-month, 30-day calendar approximation.
- The Islamic lunar calendar has 12 months totaling 354 or 355 days per year, meaning it is about 11 days shorter than the Gregorian solar year. Over 33 years, the lunar and solar calendars realign, completing a full cycle.
- The word month in English, the word Monat in German, and the word mensis in Latin all derive from roots meaning Moon, reflecting the original lunar basis of the month as a time unit.
- February has 28 days in common years and 29 in leap years. It was originally the last month of the Roman calendar year, which is why it received the leftover days. The Roman new year originally began in March, which is why September, October, November, and December preserve the Latin prefixes for 7, 8, 9, and 10 despite being the 9th through 12th months.