How this calculator works
This duration conversion defines one month as one twelfth of a 365.25-day Julian year: exactly 30.4375 days. It is an averaging convention rather than the length of a named calendar month. For a due date, contract or age, calculate between actual calendar dates instead of substituting this average.
Formula
average months = days / 30.4375; days = average months x 30.4375.
Worked example
91.3125 days is exactly 3 average months. Six average months is 182.625 days. By comparison, a particular three-month calendar span need not contain 91.3125 days.
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Frequently asked questions
Is 30 days exactly one month?
No. In this model, 30 days is approximately 0.9856262834 average months.
Why choose 30.4375 days?
It is 365.25 divided by 12, matching the Julian-year duration convention used elsewhere on this site.
Can this add calendar months to a date?
No. Named months have unequal lengths. Use a calendar date tool when the start date matters.