📆 Add or Subtract Date Calculator: Days, Weeks, Months, Years

What is the Add or Subtract Days Calculator?

Date arithmetic sounds simple but trips people up because months and years are not all the same length. Adding 30 days is not the same as adding one month, and adding one year to 29 February has no exact match in a non-leap year. This tool handles each unit the way most people expect: days and weeks step forward or back by an exact count, while months and years keep the same day-of-month where possible. The result always shows both the calendar date and the weekday, so you never have to cross-check a printed calendar.

For days and weeks the calculation is a pure count. Adding 10 days moves the calendar forward exactly 10 positions, rolling over the end of a month or year automatically. A week is just 7 days, so adding 2 weeks is the same as adding 14 days. Subtracting works the same way in reverse. Because every day is the same length, there is no ambiguity here, which is why precise legal and financial deadlines such as net 30 invoice terms or a 14-day cooling-off period are usually counted in plain calendar days.

Months and years are where care is needed. When you add one month the tool keeps the same day number if it exists in the target month. If it does not, for example 31 January plus one month, it clamps to the last valid day, giving 28 or 29 February rather than spilling into March. Adding one year to 29 February in a leap year clamps to 28 February. This last-day clamping matches how banks, contracts, and most calendar apps treat month-end dates, and it avoids the surprising jumps you get from naive arithmetic that simply adds a fixed number of days.

Leap years shape the calendar in two ways that matter here. First, February gains a 29th day in any year divisible by 4, except century years that are not divisible by 400, so 1900 was not a leap year but 2000 was. Second, because a common year is 365 days, which is 52 weeks plus 1 day, the same date moves forward one weekday each year, or two weekdays when a 29 February sits in between. That is why your birthday lands on a different day of the week each year, and why this tool always recomputes the weekday rather than assuming it.

There is also an important difference between calendar days and business days. Calendar days count every day of the week, while business days count only Monday to Friday and often exclude public holidays. A 30 calendar-day span is about four weeks, but 30 business days stretches to roughly six weeks because every weekend is skipped. This calculator works in calendar days, weeks, months, and years. If your contract, shipping estimate, or court deadline is measured in working days, count those separately or use a dedicated business-day tool, and check which holiday calendar applies to your region.

When you combine units, the order of operations can change the answer. Adding one month and then 30 days is not always the same as adding 30 days and then one month, because the month step depends on which month you are sitting in when you take it. A common, predictable convention is to apply the larger units first (years, then months, then weeks, then days), which is what most calendar libraries do. Keeping the steps in a consistent order makes results repeatable and easy to audit.

When to use it

• Finding the due date of an invoice or contract that is payable a set number of days after issue, such as net 30, net 60, or net 90.
• Working out a project deadline that is a number of weeks or months from the kickoff date.
• Counting back to find a deadline, such as the date 14 days before an event when notice is required.
• Estimating a delivery, warranty, or expiry date by adding days to an order or manufacturing date.
• Tracking a 30, 60, or 90 day milestone for onboarding plans, trial periods, or medical follow-ups.
• Answering everyday questions like what date is 100 days from today, or what was 6 months ago.

How to use the Add or Subtract Days Calculator

1. Enter the start date, or click Set start to today to use the current date.
2. Choose whether to add or subtract.
3. Type the amount you want to add or subtract.
4. Select the unit: days, weeks, months, or years.
5. Read off the resulting date and the weekday it falls on.

Formula & method

Worked examples

Examples of adding and subtracting from a start date of 19 June 2026 (a Friday)

Days in each month (common year vs leap year)

Common day-count shortcuts

Common mistakes to avoid

• Treating 30 days as one month. A month is rarely exactly 30 days. Adding 30 days to 31 January lands in early March, while adding one calendar month lands on the last day of February. Choose the unit that matches what you actually mean.
• Expecting month-end dates to keep the same day number. When the target month is shorter, the day clamps down. 31 March plus one month is 30 April, not 31 April, because April only has 30 days. This is the standard convention but it surprises people who expect a fixed day.
• Forgetting leap years. Adding one year to 29 February gives 28 February in a non-leap year. Day counts also differ: a year is 365 days most of the time but 366 in a leap year, so a 365-day jump can miss the same calendar date.
• Confusing calendar days with business days. This tool counts calendar days, which include weekends and holidays. If your deadline is in working days, 30 days becomes roughly six weeks once weekends are skipped, so count business days separately.
• Confusing inclusive and exclusive counting. This tool adds or subtracts a number of whole days from the start, so the start date itself is day zero. If a rule counts the start date as day one, you may need one fewer day than you expect.
• Mixing units in an unpredictable order. Adding one month then 30 days can differ from adding 30 days then one month, because the month step depends on which month you are in. Apply larger units first for a repeatable, auditable result.

Glossary

Start date

The date you begin from, before any days, weeks, months, or years are added or subtracted.

Unit

The kind of step you are taking: days, weeks, months, or years. A week equals 7 days.

Clamping

Reducing the day number to the last valid day of a month when the original day does not exist there, such as 31 becoming 28 in February.

Leap year

A year with 366 days, including 29 February. It occurs every 4 years, except century years not divisible by 400.

Weekday

The named day of the week (Sunday through Saturday) that a given date falls on.

Calendar day

Any day of the week, including Saturdays, Sundays, and public holidays. This calculator works in calendar days.

Business day

A working day, normally Monday to Friday and excluding public holidays. Not counted by this tool.

Net 30

A payment term meaning the amount is due 30 days after the invoice date, usually counted in calendar days.

Frequently asked questions