Date Calculator

About the Date Calculator

Add or subtract days, weeks, months or years from any date and get the result with its weekday. This is the tool for questions like "what is 90 days from today", "when is 6 months after 31 January", or "what date was 45 working days ago".

Month arithmetic is handled properly. Adding one month to 31 January gives 28 or 29 February, not 3 March — the result clamps to the end of the shorter month rather than rolling over into the next one. That is how deadlines and contracts are normally read, and it is where naive date maths most often goes wrong.

Switch on the business days option and the tool counts weekdays only, skipping Saturdays and Sundays. Useful for notice periods, payment terms and processing times that are quoted in working days.

How to use the Date Calculator

1. Pick the starting date.
2. Choose add or subtract and enter the amount.
3. Pick the unit. Business days skip Saturdays and Sundays.
4. Select Calculate date.
5. Adding a month to 31 January gives 28 or 29 February, not 3 March.

How the arithmetic works

Adding months to a month-end date

This is the case that separates a correct date calculator from a broken one. Add one month to 31 January and there is no 31 February to land on. Two behaviours are possible:

• Clamping — return 28 or 29 February, the last valid day of the target month. This tool clamps.
• Rolling over — spill into 3 March, which is what naive date libraries do.

Clamping matches how contracts and legal deadlines are normally interpreted — "one month from 31 January" means the end of February, not early March. Note that clamping is not reversible: add a month to 31 January and subtract it again and you land on 28 January, not 31. That is a property of calendar arithmetic, not a bug.

Working days calculator: what counts as a business day

Here, Monday to Friday. Saturdays and Sundays are skipped, and the count moves to the next available weekday. That matches the most common commercial convention, though it is not universal — in much of the Middle East the working week runs Sunday to Thursday.

A useful rule of thumb: business days take about 1.4 times as long in calendar terms. A "30 working day" processing time is closer to six weeks than four.

Public holidays are not included

The business-day count skips weekends only. It has no holiday calendar, because holidays vary by country, by region and by year — England and Scotland differ, US federal holidays differ from state ones, and many countries have holidays set by lunar observation. Adding a generic list would produce confidently wrong answers for most users.

Where a deadline is legally significant, check the official holiday calendar for the relevant jurisdiction and add those days manually. As a rough allowance, most countries have 8 to 15 public holidays a year, so a 90-working-day span typically absorbs three to five of them.

Common uses

• Contract notice periods — usually months, so month clamping matters.
• Payment terms — net 30 or net 60, normally calendar days from the invoice date.
• Court and filing deadlines — frequently working days, and holiday-sensitive.
• Return and warranty windows — usually calendar days from delivery.

To measure the span between two known dates instead, use the date difference calculator.

Common date counts at a glance

Some counts come up again and again: 30-day notice, 90-day returns, 180-day visa rules. Here they are from 1 January 2026, so you can see how the months fall.

Notice that 90 days is not the same as 3 months. Three months from 1 January is 1 April here only by luck; from 1 March it would be 1 June, which is 92 days. If a rule says "days", count days.

30, 90 and 180 day calculator

Set the operation to Add, type 180 and choose Days. That is all. Use Subtract to look back 180 days instead — useful when a rule asks how many days you spent somewhere "in the last 180 days".

Invoice due date calculator

Payment terms such as net 30 mean the bill is due 30 days after the invoice date. Enter the invoice date, add 30 days, and you have the due date. For net 15, net 45 or net 60, change the number.

• Net 30 — due 30 calendar days after the invoice date.
• Due in 10 business days — choose Business days so weekends are skipped.
• End of month (EOM) — the count starts at the end of the invoice month, so enter that date first.

YTD: year to date calculator

Year to date (YTD) means from 1 January up to today. To count the days, use the date difference calculator with 1 January as the start date. A handy fact: 17 October is day 290 of a normal year, so about 79% of the year has gone by then.

Pregnancy and IVF due dates

Due dates are just date arithmetic, so this calculator handles them well.

Example: a day-5 embryo transfer on 1 February 2026 gives a due date of 20 October 2026. For a human pregnancy with weeks and trimesters, the pregnancy due date calculator gives more detail.

Julian date calculator: day of the year

Many factories, food labels and military forms use a “Julian date” that simply counts the day of the year, from 1 on 1 January to 365 (or 366) on 31 December. For example, 21 October 2026 is day 294.

To find it, choose Add, start from 31 December of the year before, and count forward — or subtract 1 January from your date and add 1. This is different from the astronomer’s Julian day number and from the old Julian calendar, which runs 13 days behind today’s Gregorian calendar.

Add days calculator: quick examples

Counting from 21 October 2026: 30 days later is 20 November 2026, 60 days later is 20 December 2026, and 90 days later is 19 January 2027. Choose Add and Days above for any other count, or Business days to skip weekends.

Other ways to use this calculator

Adding days to a date solves a lot of everyday problems.

• Invoice due date calculator — add your payment terms (net 30, net 60) to the invoice date.
• IUI and IVF due date — for treatment pregnancies use the due date calculator.
• Expiration and 100-days calculator — add the shelf life or number of days to the start date.
• Date wheel calculator — a digital version of the paper date wheel: add or subtract any number of days.
• Add days to date and days from date calculator — choose Add and Days.
• Date plus months — choose Months to add calendar months instead of days.
• Business days calculator — choose Business days to skip Saturdays and Sundays.

Related guides

• IVF and Frozen Embryo Transfer Due Date

Frequently asked questions