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Date Calculator: Add or Subtract Days and Months

What is this tool?

The date calculator moves a date forwards or backwards: add or subtract days, weeks, months or years from any starting date and get the resulting date and its weekday. It answers questions like what date is 90 days from today, or what date was 6 months ago. Unlike the days between dates calculator, which counts a span, this page produces a new date, and it handles month ends and leap years instead of assuming every month has 30 days.

How it works

Days and weeks are added in calendar days. Months are added in calendar months, with the day clamped to the end of the month when the target month is shorter — so 31 January plus 1 month is 28 February in a non-leap year, not 3 March. Years are added as 12 months, so a February 29 date moves to February 28 in a non-leap year.

Worked example

Worked example: 1 January 2026 plus 90 days is 1 April 2026, a Wednesday. And 31 January 2026 plus 1 month is 28 February 2026, because February 2026 has only 28 days.

When to use it

Moving a date forward or backward is a daily task in planning, and it comes up in far more settings than the name suggests. The most common use is finding a future date from a starting point plus a fixed interval. A follow-up in 30 days, a review in 90 days, a renewal in 6 months and an anniversary in 10 years are all the same kind of calculation, and each one needs an exact date and weekday rather than a rough sense of next month or next quarter. From 1 January 2026, adding 30 days gives 31 January 2026, a Saturday, adding 60 days gives 2 March 2026, a Monday, and adding 365 days gives 1 January 2027, a Friday. Those three results look simple, but none of them can be found reliably by counting months on fingers, because the months involved have different lengths.

Going backwards is equally common. Counting back from an event to find when preparation should start, when a booking window opens, or when an earlier step must have happened, all require the same tool in reverse. From 1 January 2026, subtracting 90 days gives 3 October 2025, a Friday, which is a useful anchor for anything that needs three months of preparation before the new year. Counting back by months is just as useful. From 20 March 2026, subtracting 6 months gives 20 September 2025, a Saturday, which shows how a half-year step lands on the same day of the month when the months allow it, and on the month end when they do not.

Anniversaries and recurring dates are a third use. Adding years to a starting date gives the same calendar date in a future year, with the weekday shifting, which is exactly what anniversary planning needs. From 15 May 1990, adding 10 years gives 15 May 2000, a Monday, and the same step works for any starting date and any number of years. The same logic applies to monthly recurrences, such as a payment, a meeting or a check that repeats on the same day each month. The tool handles the months where that day does not exist, which is the point where manual counting most often goes wrong.

Finally, the tool is useful for checking other calculations. If a span between two dates looks wrong, adding the span back to the start date should return the end date, and if it does not, one of the two figures is wrong. This round-trip check is the simplest way to catch a one-day or one-month error before it affects a plan. It also helps when a date has been quoted in words rather than numbers, because converting a phrase like three months from now into an exact date removes the ambiguity about which months were meant and whether the end of the month was involved.

Step by step

Start with the starting date, and enter it exactly. The starting date is the anchor for everything that follows, so a mistake here shifts the result by the same amount. Use the date picker when it is available, because it removes the risk of swapping day and month, and read the date back once if you type it. Then choose the amount and the unit together, because the unit changes the meaning of the amount completely. An amount of 3 in days is a short step, an amount of 3 in weeks is 21 days, an amount of 3 in months is roughly a quarter of a year, and an amount of 3 in years is a long step. The tool treats each unit by its own rule, and mixing them up is the fastest way to get a wildly wrong date.

Next, choose the direction, and remember that direction and a negative amount do the same thing, so use only one of them. Choose Add to move forward and Subtract to move backward, and keep the amount positive. Entering a negative amount with Subtract selected would move in the opposite direction from the one intended, because the two negatives cancel out. If the tool you are using does not accept negative amounts, that is a safeguard, not a limitation, and it exists to prevent exactly this error. Once the direction is set, run the calculation and read both parts of the result: the date itself and the weekday.

Then check the result against the month-end rule if the unit was months or years. When the starting day does not exist in the target month, the result is clamped to the last day of that month. From 31 August 2026, adding 1 month gives 30 September 2026, a Wednesday, because September has only 30 days. From 31 October 2025, adding 1 month gives 30 November 2025, a Sunday, for the same reason. From 28 February 2026, adding 1 month gives 28 March 2026, a Saturday, because March has 31 days and the 28th exists there. Clamping is not rounding, and it is not spilling into the next month. It is the standard calendar convention, and knowing it in advance prevents surprise when a monthly recurrence lands on the 30th instead of the 31st.

Finally, verify the result by counting back. Subtracting the same amount from the result should return the starting date in most cases, with one important exception. When clamping has occurred, counting back may not return the original day, because the lost days cannot be recovered. From 31 January 2026, adding 1 month gives 28 February 2026, but subtracting 1 month from 28 February 2026 gives 28 January 2026, not 31 January. That asymmetry is not an error. It is a consequence of months having different lengths, and it is worth knowing before relying on a round trip for a month-end date.

Common mistakes

The most persistent mistake is treating a month as 30 days. Adding 30 days and adding 1 month are different operations, and they give different dates from the same starting point. From 31 January 2026, adding 30 days gives 2 March 2026, a Monday, while adding 1 month gives 28 February 2026, a Saturday. The two results differ by two days, and the gap grows or shrinks depending on which months are involved. The rule to remember is that a month is a calendar unit, not a fixed number of days, and the correct operation depends on what was actually meant. If a deadline says 30 days, add days. If it says 1 month, add a calendar month. Using one when the other was meant is how deadlines are missed by days.

A second mistake is forgetting leap days when adding years. From 29 February 2024, adding 1 year gives 28 February 2025, a Friday, because 2025 is not a leap year and February has only 28 days. People who expect 29 February 2025 will search in vain for a date that does not exist, and people who push the result to 1 March will be a day late. The same care applies to adding years to any February date near the month end, because the leap-year pattern changes the length of February every four years, with century exceptions. The tool applies the full rule, including the century rule, so the result can be trusted, but the expectation needs to match the calendar.

A third mistake is assuming the weekday stays the same after adding months or years. It does not. From 15 June 2026, adding 6 months gives 15 December 2026, a Tuesday, and the weekday of the result depends on the number of days in the intervening months, not on the starting weekday alone. From 15 May 2026, adding 3 months gives 15 August 2026, a Saturday. Neither result shares the starting weekday, and neither should be expected to. Only adding a whole number of weeks preserves the weekday, because a week is exactly seven days. Adding 52 weeks to 1 January 2026 gives 31 December 2026, a Thursday, which is not the same weekday either, because 52 weeks is 364 days, one day short of a full year. Only a multiple of seven days preserves the weekday, and a year is not a multiple of seven.

Finally, watch out for double negatives and for counting the starting day. Subtracting with a negative amount, or adding with the starting day counted as day one, both shift the result by the amount of the error, and both are easy to make when working quickly. The cleanest habit is to keep amounts positive, choose the direction explicitly, and treat the starting date as day zero. From 1 January 2026, adding 90 days gives 1 April 2026, a Wednesday, counting 90 steps forward from the starting date, not 90 days including it. If a task counts the starting day as day one, add one to the amount or state the convention clearly, so the next person reading the result knows which convention was used.

More worked examples and notes

Start with the short day-based examples, because they are the ones people check most often. From 1 January 2026, adding 30 days gives 31 January 2026, a Saturday. Adding 60 days gives 2 March 2026, a Monday, which crosses February and shows why the month length matters: January contributes 30 days after the starting date, February contributes 28, and the remaining 2 days land in March. Adding 90 days gives 1 April 2026, a Wednesday, which crosses three month boundaries. Adding 365 days gives 1 January 2027, a Friday, which is one full non-leap year later. Each of these results can be checked by counting forward on a calendar, and each one shows a different month-length effect.

Next, consider the week-based example, which is often misunderstood. Adding 52 weeks to 1 January 2026 gives 31 December 2026, a Thursday, because 52 weeks is 364 days, and 2026 has 365 days. People who expect a year of weeks to land on 1 January 2027 are one day out, because a calendar year is not a whole number of weeks. The same point applies in reverse. Subtracting weeks from a date moves the weekday not at all, because every week is seven days, but subtracting a year moves it by one or two days depending on leap days. Keeping weeks and years as separate units, rather than converting between them casually, avoids this class of error entirely.

Month-based examples show the clamping rule in action. From 31 August 2026, adding 1 month gives 30 September 2026, a Wednesday. From 31 October 2025, adding 1 month gives 30 November 2025, a Sunday. From 15 June 2026, adding 6 months gives 15 December 2026, a Tuesday, where no clamping is needed because the 15th exists in December. From 15 May 2026, adding 3 months gives 15 August 2026, a Saturday. From 28 February 2026, adding 1 month gives 28 March 2026, a Saturday. Together, these examples show that clamping only happens when the starting day is beyond the end of the target month, and that ordinary mid-month dates move cleanly from month to month.

Year-based and backward examples complete the picture. From 15 May 1990, adding 10 years gives 15 May 2000, a Monday. From 29 February 2024, adding 1 year gives 28 February 2025, a Friday. From 1 January 2027, subtracting 1 year gives 1 January 2026, a Thursday. From 1 January 2026, subtracting 90 days gives 3 October 2025, a Friday. From 20 March 2026, subtracting 6 months gives 20 September 2025, a Saturday. The backward examples mirror the forward ones, with the same month-end rule applying in reverse, and the same need to read both the date and the weekday. Taken together, these examples cover the four units and both directions, which is the full range of what the tool does, and any new calculation can be checked against the nearest example here before it is relied upon.

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Frequently asked questions

Written and checked against the calculator above. Last updated: 21 October 2026. Calculation runs entirely in your browser — no date is sent to a server. This tool gives calendar results only, not medical, legal or financial advice.