A full month runs day-number to the same day-number
The count is complete months, not an average. A month completes when the calendar reaches the same day number in a later month. If the end date has not yet passed the start day, that final month is still in progress, so it does not count — you are left with a whole-month figure plus some leftover days.
The three things people get wrong
- Months are not fixed-length. They run 28 to 31 days, so "3 months" is 89 to 92 actual days depending on which months it spans. A day count and a month count answer different questions.
- Short target months still complete. A start day of the 31st completes into a 30-day month on that month's last day. The missing 31st does not push the count out by a month.
- Leftover days are not a fraction of a month. After the last whole month, the remainder is counted in days, not rounded up. A result like "4 months and 12 days" is exact; "4.4 months" is only an approximation using an average month of 30.44 days.
Where a month count is the one that matters
- Leases and rent. A tenancy is billed by whole months from a fixed day, so a term stated in months, not days, is what the contract counts.
- Loans and finance. A 30-year mortgage is exactly 360 monthly payments; a 15-year loan is 180. Each month is one payment regardless of its length.
- Age of an infant. Age is given in months up to about two years, counted the same day-to-day way.
- Warranties and subscriptions. A "24-month" cover period ends on the matching day two years on, not after a fixed number of days.
Contracts sometimes distinguish "30 days" from "one month" on purpose. When a deadline is written in days, count days; the two are not interchangeable.
Common questions
What counts as a full month?
A full month runs from a day number to the same day number in a later month. From the 15th of one month to the 15th of the next is one full month, whatever the days in between. If the end day falls short of the start day, that month has not completed yet and is not counted.
Is three months always 90 days?
No. Months vary from 28 to 31 days, so three consecutive months run anywhere from 89 to 92 days. December plus January plus February is about 90, but April plus May plus June is 91. Count whole months when the term is written in months, and days when it is written in days.
How does this handle the end of the month?
When the start day does not exist in the target month, the month still counts once its last day is reached. From the 31st of a 31-day month to the 30th of a 30-day month completes that month, because the 30th is the last day available.
