Date Calculator

About this tool

Date arithmetic is deceptively tricky. Adding 30 days to March 15 is straightforward, but counting 90 days from today means stepping through months of unequal length. This date calculator does the counting for you. Choose a start date (it defaults to today), pick Add or Subtract, enter a whole-number amount, and choose the unit — days, weeks, months or years — then click Calculate. The result is shown three ways: as an ISO date (YYYY-MM-DD), in your local date format, and as a full date with the day of the week, which is useful for spotting whether a deadline falls on a weekend. Months and years follow JavaScript calendar rules, so a day that does not exist in the target month rolls forward: January 31 plus one month gives March 3 in a non-leap year, and February 29 plus one year gives March 1. Everything is calculated in your browser.

Why use it

• Add and Subtract in Both Directions: Switch between moving forward into the future and back into the past with one dropdown, using the same amount and unit.

• Four Units: Offset by days, weeks, months or years, which covers notice periods, contract terms, probation periods and annual renewals.

• Day-of-week Display: The result includes the weekday name, which helps you immediately spot whether a calculated deadline lands on a Saturday or Sunday.

• Three Date Formats: The result appears as an ISO date for spreadsheets and code, in your local format, and written out in full, so you can use whichever you need.

• Private Browser-side Calculation: Dates are sometimes sensitive — court dates, medical appointments, HR deadlines. All arithmetic happens locally in your browser and no date is sent to a server.

How to use

1. Enter the start date — it is set to today by default
2. Select the Operation: Add (+) or Subtract (-)
3. Enter a whole-number Amount
4. Choose the Unit: Days, Weeks, Months or Years
5. Click Calculate and read the ISO date, local date and weekday

When it helps

• When a contract specifies a deadline of 90 days from signing and you need the exact calendar date
• When a project milestone is defined as 6 months after kickoff and you need to schedule it correctly
• When scheduling a renewal, warranty, or subscription review and you want the exact expiry date
• When an HR team needs to calculate the end of a probationary period from a hire date
• When a legal filing must be submitted within 30 days of a court order and you need the precise due date
• When planning travel and you want to know what date falls a specific number of days before or after departure

Examples

Tips

• Use the Business Days Calculator if your deadline excludes weekends and holidays — this tool counts all calendar days
• Be careful adding months to the 29th, 30th or 31st — short months roll the result into the next month
• For a combined offset, run one unit at a time and feed each result back in as the new start date
• Check the weekday in the result to spot weekend landings before committing to a date
• For a 90-day deadline, use Days rather than 3 Months — the two can differ by a day or two

Frequently Asked Questions

Glossary

Calendar Day

Any day on the Gregorian calendar, including weekends and public holidays. Calendar-day counting is the default for most date arithmetic unless working-day rules apply.

Weekday

The named day of the week (Monday through Sunday) associated with a specific calendar date. Knowing the weekday of a calculated date is essential for scheduling and deadline management.

Month-end Roll

The behaviour when adding months produces a day that does not exist in the target month. Clamping sets the result to the last valid day; overflow rolls into the next month. Most calendar systems clamp.

Leap Year

A year with 366 days in which February has 29 days. Gregorian leap years occur every 4 years, except for century years not divisible by 400. Date calculators must handle leap years to avoid incorrect results near February 28–29.

ISO 8601 Date

The international standard date format YYYY-MM-DD (e.g., 2026-05-07). It is unambiguous across locales, sorts correctly as a string, and is the recommended format for data exchange and scheduling.

Date Arithmetic

The process of adding or subtracting time intervals from calendar dates. It must account for month-length variation, leap years, and DST transitions when dealing with timestamps rather than dates.