Month (mo)

Definition

About 30.44 days on average — a twelfth of a 365.25-day year, not a calendar month (which runs 28 to 31 days).

History

The month began as the period of the lunar phases, about 29.5 days, but calendar months were fixed at 28 to 31 days to fit twelve into a year. For conversions an average month is taken as a twelfth of a year, about 30.44 days.

Where it is used

Months organise the calendar, billing cycles and subscriptions, and count durations such as pregnancies and projects.

Typical uses include durations, billing and subscriptions.

Conversion pages

These pages convert month to or from another unit in time.

No conversion pages use this unit yet.

Other Time units

• Second (s)
• Minute (min)
• Hour (h)
• Day (d)
• Week (wk)
• Year (yr)

Sources

• Julian year (IAU/ISO) IAU/ISO 80000-3 Julian year: 1 a = 365.25 d exactly, a convention for expressing durations. The average month here is that year divided by 12. Neither is an SI unit. · Dataset reviewed Sep 18, 2026