Month Calculator

whole months between = full calendar months from start to end; landing date = the same day number in the target month, clamped to that month's last day when there is no such day

startDate

The first date, and also the date that gets shifted

endDate

The second date; swapping the two gives the same month count

months

How many whole months to shift by; a negative value moves the date backwards

monthsBetween

Whole months only — 1 January to 31 December is 11, not 12, because one day is missing

remainderDays

The days past the last whole month, which can be anything from 0 to 30

resultDate

Where startDate lands after the shift, with month end clamping applied

Reach for this month calculator when a term is quoted in months rather than days: a lease, a warranty, a notice period, a subscription, a payment plan. Whole months are the right unit for those, and the leftover days are what tells you whether a period is really complete — a 12-month term ending a day short is not 12 months. The six-month default shows the shape of both answers at once. It cannot give you a fractional month: half a month is not a landing date the calendar can produce, so the month count has to be a whole number.

1. 1 January to 19 September 2026, and six months on from the start

   1. From 1 January 2026, eight whole months take you to 1 September 2026
   2. 19 September is 18 days past that, so the leftover is 18 days and the month count stays at 8
   3. The 18 matters: without it, 1 September and 19 September would read the same
   4. Shifting the start date six months forward gives 1 July 2026, a Wednesday

   The leftover days are the whole reason this page reports four numbers instead of one. Two intervals a fortnight apart in length can both read eight months.

2. One month on from 31 January 2026

   1. February 2026 has 28 days, so there is no 31 February for the shift to land on
   2. The landing date clamps to the last day of that month: 28 February 2026
   3. The gap from 31 January to 1 March is one whole month plus one day
   4. That leftover is 1, not minus 30 — subtracting day numbers would give the wrong sign and the wrong size

   Month end clamping and the leftover days are the same rule seen from two sides. Both are wrong in a tool that implements a month as thirty days.

3. One month on from 31 January 2024

   1. 2024 is a leap year, so February has 29 days
   2. The clamp target moves with it: the landing date is 29 February 2024, a Thursday
   3. The month count and the leftover are unchanged at 1 and 1
   4. Only the landing date differs from the 2026 example, and it differs by exactly one day

   Same instruction, same day of the month, one day apart in the answer. A leap year is the only thing that separates these two examples.

4. A full year, read in months

   1. 1 January 2026 to 1 January 2027 is exactly 12 months, so the leftover is 0 rather than 1
   2. The day count over that span is 365, but the month count is a clean 12
   3. Shifting the start by 12 months returns the same day number a year later: 1 January 2027
   4. 1 January 2027 is a Friday

   Twelve months and 365 days describe the same span here, and they will not agree for 2028 — which is why the two units are reported separately rather than converted.

Whole months are counted by calendar months, not by dividing the day count by an average month length. That means the answer depends on where the months fall: 1 January to 1 February is one month, and so is 1 February to 1 March, even though the first is 31 days and the second is 28. Month end clamping follows the convention spreadsheet software uses — landing on the last day of the target month rather than rolling into the next one — so a shift from the 29th, 30th or 31st can move fewer days than a whole month. The month count must be a whole number, because a fractional month has no landing date; and swapping the two dates does not reverse the sign, since the size of a gap is the same in both directions.

How many months between 1 January and 19 September 2026?

Eight whole months plus 18 days. Eight months from 1 January lands on 1 September, and 19 September is 18 days later. Counting it as nine months would be wrong in the other direction: a ninth month is not complete until 1 October. Every answer on this page is split the same way, so the two numbers always add up to the real gap.

How do I add months to a date?

Enter the start date and the number of months, and read the landing date. Six months on from 1 January 2026 is 1 July 2026, and the day number is preserved when the target month is long enough. For a negative number the shift runs backwards, so minus six months from 1 July 2026 returns 1 January 2026.

What happens when the target month is too short?

The landing date clamps to the last day of that month. One month on from 31 January 2026 is 28 February 2026, and one month on from 31 January 2024 is 29 February 2024. This month end clamping is the rule spreadsheets use, and it is why a one-month shift can move 28 days instead of 31.

Why is 1 January to 31 December only 11 months?

Because the twelfth month is not finished. A month runs from a day number to the same day number in the next month, so the twelfth month closes on 1 January. From 1 January to 31 December 2026 the whole-month count is 11 with 30 leftover days, while the day count is 364.

Does the order of the two dates matter?

Not for the month count. Swapping them gives the same answer, because the size of the gap between two dates does not depend on which one you read first — 1 January to 19 September and 19 September to 1 January are both eight months and 18 days. The only input whose sign matters is the number of months to shift, which is applied to the first date.

• relativedelta — the dateutil module — dateutil project documentation
• The Gregorian calendar — Claus Tøndering, Calendar FAQ