Date Calculator: Add or Subtract Days, Months & Years

What is a date calculator?

A date calculator finds the date that falls a given number of days, weeks, months or years before or after another date. Use this date calculator to work out a deadline 90 days from today, a notice period ending one month after resignation, a warranty that expires two years after purchase, or a delivery date 10 working days from an order. It handles leap years and month ends correctly, shows the weekday, the ISO week number and the day of the year, and has a business-days mode that skips weekends and any holidays you list.

All calculations use calendar dates only — no time of day, no time zones — so the answer is the same wherever you are. To count the days between two dates, use our days between dates calculator.

How to use the date calculator

1. Pick the start date; it defaults to today on your device.
2. Choose Add or Subtract.
3. Enter years, months, weeks and/or days on the Add / subtract tab. You can combine them, e.g. 1 year, 2 months and 10 days.
4. Or switch to Business days, enter the number of working days, choose a Saturday-Sunday or Sunday-only weekend, and paste any holidays as YYYY-MM-DD dates.
5. Read the result — full date with weekday, ISO date, ISO week, day of the year and the number of calendar days between the two dates. Use Copy result to paste it elsewhere.

How the date calculator works

Years and months first, then weeks and days. Years are treated as 12 months. The calculator moves the month and year, keeps the same day number if it exists, and otherwise uses the last day of the target month (month-end clamping). Weeks (× 7) and days are then added to that date.

• 31 January 2026 + 1 month = Saturday, 28 February 2026 (2026 is not a leap year).
• 31 January 2028 + 1 month = Tuesday, 29 February 2028 (2028 is a leap year).
• 29 February 2024 + 1 year = Friday, 28 February 2025.
• 31 March 2026 − 1 month = Saturday, 28 February 2026.
• 15 August 2026 + 100 days = Monday, 23 November 2026.

Because of clamping, adding one month and then subtracting one month does not always return the original date: 31 January + 1 month = 28 February, and 28 February − 1 month = 28 January. This is the same rule used by most spreadsheet functions such as EDATE.

Leap years follow the Gregorian rule: a year divisible by 4 is a leap year, except century years, which must be divisible by 400. So 2000 and 2024 are leap years, 1900 and 2100 are not.

ISO week numbers follow ISO 8601: weeks start on Monday, and week 1 is the week containing the year’s first Thursday. Dates in late December can belong to week 1 of the next year and early January dates to week 52 or 53 of the previous year — for example, 31 December 2024 is in ISO week 2025-W01.

Business days mode

The calculator steps forward (or backward) one day at a time, counting only working days — days that are not Saturday or Sunday (or only not Sunday, for a 6-day week) and are not in your holiday list. The start date itself is not counted, which matches the usual meaning of “within 10 working days”.

Example: from Wednesday, 14 October 2026, adding 10 business days with a Saturday-Sunday weekend gives Wednesday, 28 October 2026 (4 weekend days skipped). If you add 20 October 2026 as a holiday, the result moves to Thursday, 29 October 2026. Adding 1 business day to a Friday gives the following Monday.

No holidays are built in, because public and bank holidays differ by state, organisation and year in India. Paste the list that applies to you — for example, from your state government or company holiday calendar.

Common uses

• Notice periods, probation end dates and contract renewals (“3 months from joining”).
• Payment due dates, loan EMI dates and invoice terms such as “net 30”.
• Delivery and dispatch estimates in working days.
• Exam, visa and project deadlines; checking which weekday a date falls on.
• Finding the ISO week number used in production schedules and sprint planning.

Tips

• “30 days” and “1 month” are not the same: from 1 February 2026, 30 days is 3 March, while 1 month is 1 March.
• If a legal or contractual deadline matters, check whether the start day is counted and whether holidays shift the last day.
• To find the date before an event, choose Subtract — e.g. 45 days before a trip to know when to apply for a visa.

Limitations and notes

The tool uses the proleptic Gregorian calendar for all years from 1 to 9999 and does not model historical calendar changes (such as the 1752 switch in Britain). Business-day results depend on the holidays you enter. The default start date is taken from your device’s clock.

Frequently Asked Questions

What date is 1 month after 31 January?

It is the last day of February: 28 February in a normal year and 29 February in a leap year. When the target month has fewer days, the date is moved to the last day of that month.

How do I add business days to a date?

Switch to the Business days tab, enter the start date and the number of working days, and choose your weekend. The calculator skips Saturdays and Sundays (or only Sundays) and any holidays you list. The start date itself is not counted.

What is 100 days from 15 August 2026?

Monday, 23 November 2026.

What is an ISO week number?

ISO 8601 numbers weeks from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year. A year has 52 or 53 ISO weeks, and dates at the very start or end of a year can belong to the neighbouring ISO year.

Does the date calculator account for leap years?

Yes. It uses the Gregorian rule: years divisible by 4 are leap years except century years not divisible by 400. For example, 29 February 2024 plus 1 year gives 28 February 2025.

Will I get a different answer in a different time zone?

No. The calculator works on calendar dates only, without times, so the result is the same in every time zone. Only the default start date (today) comes from your device clock.

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