Adding Months to a Date: The Month-End Rule

Why months are tricky

Days and weeks are fixed lengths, but a month is not. It can be 28, 29, 30 or 31 days long. So when you add a month to a date, you cannot simply add a fixed number of days. You add a calendar month, and the length of that month depends on which one it is.

The result is easy in the ordinary case. One month after 22 October 2026 is 22 November 2026. Three months after it is 22 January 2027, a Friday, and twelve months after it is 22 October 2027. The day number stays the same, and only the month moves on.

The same-day rule

The rule is to keep the day number and move the month. A date such as the 15th of a month becomes the 15th of the target month. This works for every month, because every month has at least 28 days, so any day from 1 to 28 exists in all of them.

Adding several months works the same way. Six months after 15 March is 15 September, and eighteen months after it is 15 September of the following year. A year is simply twelve months, and the date comes back to the same day and month.

The month-end problem

Trouble starts with the 29th, 30th and 31st, because the target month may not have that day. One month after 31 January should be the 31st of February, which does not exist.

The standard solution is to use the last day of the target month. One month after 31 January 2026 is 28 February 2026. In 2024, a leap year, one month after 31 January is 29 February 2024. One month after 30 January is the same: 28 February in 2026 and 29 February in 2024.

The same rule applies in the other direction. One month before 31 March 2026 is 28 February 2026, and one month before 31 May is 30 April. The result never spills into the next month, because the rule always stays within the target month.

Three months after 30 November 2026 is 28 February 2027, again the last day of the month, because February has no 30th.

One step or several steps

A subtle point is that the order matters when you add months in stages. Add two months to 31 January 2026 in a single step and you reach 31 March 2026. Add one month twice, and you reach 28 February first, then 28 March.

The single-step method is more accurate because it always counts from the original date. The step-by-step method loses the original day number as soon as it is shortened. For that reason, a calculator should add the whole number of months at once, and this one does.

If you need a series of monthly dates, such as instalments on the 31st, calculate each one from the first date, with its own number of months added, not from the previous result.

Months are not 30 days

A month and 30 days are different, and the difference matters. Thirty days after 31 January 2026 is 2 March 2026, and one month after is 28 February 2026. Over six months the gap grows: six calendar months after 31 August 2026 is 28 February 2027, which is 181 days later, while 180 days later is 27 February 2027.

If a document says a month, use the calendar. If it says 30 days, use 30 days. If it is not clear, find out which was meant, because the two can differ by up to three days.

Businesses sometimes use a simplified year of twelve 30-day months, called 30/360, in financial calculations. It is a convention, and it is only correct where it is stated.

How spreadsheets and the law handle it

Spreadsheet programs have a function for this. EDATE in Excel and in Google Sheets adds a number of months to a date and returns the last day of the month if the day does not exist, which matches the rule here. A negative number of months subtracts.

Legal rules often follow the same pattern. The European Union regulation on time limits, 1182/71, says that a period in months ends on the day that has the same date as the day of the event, and if the final month has no such day, it ends on the last day of that month. National laws differ in their details, so read the rule that applies to you.

Billing systems have their own approach. A subscription that starts on the 31st may bill on the last day of each month, or on the 1st. If you manage billing, decide on one rule and state it.

Using the calculator

Open the Add or subtract tab, enter the start date and type the number of months. You can combine it with years, weeks and days. The calculator adds the years and months together first, applies the month-end rule, and then adds the weeks and days.

To see the rule in action, try 31 January with one month, then try 31 January in a leap year such as 2028. The result is the last day of February both times.