Months Between Dates Calculator

How it is calculated

Both dates must be real calendar dates written as YYYY-MM-DD, and the end date cannot be earlier than the start (the same day gives 0). A month counts as complete on the same day number of the next month; if that month is shorter, the last day of the month is used, so 31 January to 28 February is one whole month. Whole months are the largest count whose anchor date does not pass the end date, and the leftover is the extra days from that anchor to the end date. Decimal months are separate: total days ÷ 30.4375, where 30.4375 is the average month length (365.25 ÷ 12). The total days figure is the exact day count, with leap days included, and the end date itself is not added as an extra day. Because the calendar count and the decimal figure use different rules, they can disagree slightly.

Common mistakes

• Typing the date in a local order such as 15/01/2024. The tool needs year-month-day, 2024-01-15, and an impossible date like 2025-02-30 gives no result.
• Expecting a month-end start to behave like PHP or spreadsheet date maths that rolls into the next month. Here 31 January to 28 February counts as 1 month and 0 days, but 31 January to 30 March is 1 month and 30 days, because 31 March has not been reached.
• Comparing the decimal months with the whole months. 15.15 months is a day count divided by an average month, so it will not exactly match 15 months and 5 days.
• Putting the later date first. The end date cannot be before the start date; there is no result, and the tool does not swap them for you.
• Using the decimal months for rent or interest proration without checking your contract. Many agreements count calendar months or actual days, not 30.4375-day months.