Is 30 days the same as a month?
No. A calendar month runs 28, 29, 30 or 31 days, so 30 days usually lands on a different date. Start in a 30-day month and the two match. Start in February and 30 days overshoots by one or two days. Start in most other 31-day months and it falls a day short.
How long is a month, in days?
It depends on the month. Seven months have 31 days, four have 30, and February has 28, or 29 in a leap year. Averaged over the 400-year cycle of the Gregorian calendar, a month comes to 30.436875 days. That's the year length of 365.2425 days divided by 12, and it's why no whole number of days equals "a month."
The leap year rule behind that average is short. A year divisible by 4 is a leap year, except century years, which count only when divisible by 400. NIST states it this way: century years "are considered as leap years only if they are evenly divisible by 400." So 2000 was a leap year and 2100 won't be.
When do 30 days and one month disagree?
Most of the time, and the gap grows with longer spans. Here's what each gives from a few start dates in 2026, with one month clamped to the last day when the date doesn't exist:
| Start date | Plus 30 days | Plus 1 month | Gap |
|---|---|---|---|
| January 31 | March 2 | February 28 | 2 days |
| February 1 | March 3 | March 1 | 2 days |
| February 15 | March 17 | March 15 | 2 days |
| April 10 | May 10 | May 10 | none |
| July 15 | August 14 | August 15 | 1 day |
From any day of a 30-day month (April, June, September or November) the two always agree. Elsewhere they rarely do. The 31st of March, May, August or October also matches, because one month later is cut back to the 30th. February is the worst case, because 30 days is longer than the whole month.
Does the gap grow over 3, 6 or 12 months?
Yes, and it's the reason "90 days" and "3 months" aren't interchangeable on a deadline. Three calendar months run anywhere from 89 to 92 days, depending on where they start:
- January 1 plus 90 days is April 1, the same as 3 months. That's luck: January, February and March add up to exactly 90 days in a common year.
- July 1 plus 90 days is September 29. Three months from July 1 is October 1, two days later.
- July 1 plus 180 days is December 28. Six months from July 1 is January 1, four days later.
- March 1, 2027 plus 360 days is February 24, 2028. Twelve months from March 1, 2027 is March 1, 2028, six days later.
If a document says one and you count the other, you can be off by up to four days on a six-month span, and by five or six on a year.
What about a month that starts on the 31st, where the next month has no 31st? That's a rule rather than a sum, and different tools pick different rules. Does the end date count? covers it, and the date calculator states its rule below the result.
Which should you use?
Use whichever unit the source uses, and don't convert.
If a return window, a trial or a notice period says "30 days," count days. If it says "one month," move the calendar date. Converting one into the other is where the error creeps in, since there's no exact conversion to make. When the wording matters and you can't tell which was meant, ask whoever wrote it. They set the terms, so they can say which unit they meant.
For counting, the date calculator moves a date by days, weeks, months or years, and the days between dates calculator counts the days in a span you already have. Both live in our date and time tools.
Common questions
Is a month 4 weeks?
No. Four weeks is exactly 28 days, so it only matches February in a common year. Every other month is 1 to 3 days longer than four weeks (February in a leap year by one). Over a year that adds up: 13 four-week blocks make 364 days, against 12 months making 365 or 366.
How many days is 3 months?
Between 89 and 92, depending on the start date. Three months from February 1 is 89 days in a common year. Three months from July 1 is 92.
How many days is 6 months?
Between 181 and 184. Six months from January 1, 2026 is 181 days. Six months from July 1, 2026 is 184.
Is 30 days from today the same date next month?
Reliably only if this month has 30 days. From most days of a 31-day month, 30 days lands one date earlier than the same date next month, and from the 1st it lands on the 31st of the same month. In February it lands one or two dates later. Late January is the odd one out, because February is so short.