Common Years to Months (yr (365) to mo)

1 common year equals 11.99203269 months (rounded).

1 common year = 11.99203269 months

How to convert common years to months

To convert common years to months, multiply the number of common years by 11.99203269. The factor is rounded to 10 significant digits. To go the other way, multiply months by 0.08338869863 (rounded).

months = common years × 11.99203269

Example: 10 common years = 119.9203269 months.

Months and years are not constant. This site uses the average Gregorian year of 365.2425 days, so one 'year' converts to 365.2425 days, not 365; use 'common year' for exactly 365 days.

About these units

Common Year (yr (365))

CalendarCurrent unit

A calendar year of exactly 365 days, without a leap day.

Calendar makers needed whole numbers of days, so the common year is simply 365 of them, or 31,536,000 seconds at 86,400 seconds per day. It makes up 303 of every 400 years in the Gregorian calendar. Because 365 is 52 weeks plus one day, a common year begins and ends on the same weekday, and each date falls one weekday later than the year before.

The trouble is that the Earth takes about 365.24 days to complete a cycle of seasons, so a common year runs roughly 5 hours, 48 minutes and 46 seconds short of the tropical year. The shortfall adds up to a full day in about four years, which is why leap years exist. NIST lists the 365-day year as an exact conversion, while its tropical and sidereal years are measured quantities that differ in the fifth significant figure.

1 common year = 31,536,000 seconds (exactly).

Sources: Wikipedia: Common year; NIST SP 811 Appendix B.8: factors for units outside the SI

Month (mo)

CalendarCurrent unit

One twelfth of an average Gregorian year: 30.436875 days. Calendar months actually range from 28 to 31 days.

Month and Moon are cognates, and the unit began as the Moon's cycle of phases, about 29.53 days from one new moon to the next. The trouble is that 12 of these fall short of a solar year, so calendars have had to choose. The Islamic calendar stays purely lunar, so its year is about 11 days shorter than the solar year, while the Hebrew calendar is lunisolar, using a 19-year cycle with an extra month. Even the start of a lunar month is still debated between observing the crescent and calculating it.

Gregorian months are a compromise that drifted far from the Moon. Their lengths run from 28 to 31 days, and this site uses their mean of 30.436875 days, one twelfth of the average year. The names also keep Roman history: September to December preserve the Latin numerals for seven to ten, since they were originally the seventh to tenth months, and Quintilis and Sextilis were renamed for Julius Caesar and Augustus. When a contract says month, check whether it means a calendar month or thirty days.

1 month = 2,629,746 seconds (exactly).

Sources: Wikipedia: Month; Wikipedia: Lunar calendar

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