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EVOA Toolbox

Date Calculator

Find the difference between two dates, add or subtract days, weeks, months, years or business days, get the weekday and ISO week, and calculate age.

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What this tool does

Four everyday date tasks in one place. Work out the exact difference between two dates in years, months and days along with total days, weeks, hours and business days. Add or subtract days, weeks, months, years or business days from a starting date. Look up the weekday, ISO week number and day of the year for any date. Or calculate an age and the days until the next birthday.

Month arithmetic follows calendar rules: adding one month to 31 January gives the last day of February (28th or 29th in leap years), and adding a year to 29 February gives 28 February in common years. Business days count Monday to Friday and skip weekends; public holidays are not included because they differ by country and year.

All calculations use plain calendar dates with UTC arithmetic, so daylight-saving changes can never shift a result by a day. Hours are shown as days × 24 for the same reason. Nothing is sent from your device.

How to use it

  1. 1Choose a tab: Difference, Add or subtract, Date info, or Age.
  2. 2Enter the dates. The result updates as you type.
  3. 3For differences, optionally include the end date itself in the count.
  4. 4For adding, choose the unit (including business days) and whether to add or subtract.
  5. 5Copy the resulting date if you need it elsewhere.

Supported formats

Dates are entered with the browser date picker (YYYY-MM-DD) for years 0001 to 9999.

Privacy

This tool runs in your browser. The data you provide is processed on your device and is not sent to our servers.

Limitations

  • Business days exclude only Saturdays and Sundays. Public and company holidays are not considered.
  • Dates are calendar dates without time or time zone. Total hours is days × 24 and ignores daylight-saving hours.
  • The Gregorian calendar is applied to all years, including those before its adoption in 1582.
  • A 29 February birthday is treated as 28 February in non-leap years for the next-birthday count. Legal conventions differ by country.

FAQ

How are months and days in a difference worked out?

The tool counts whole years and months from the earlier date, then the remaining days. Jan 31 to Mar 1 in a leap year is 1 month and 1 day because Jan 31 plus one month lands on Feb 29.

Does the difference include the end date?

Not by default: Monday to Tuesday is 1 day. Turn on Include end date to count both days, which gives 2.

What is an ISO week?

ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year. Around New Year, a date can belong to week 52 or 53 of the previous year or week 1 of the next.

How do business days handle weekends at the start?

Adding one business day to a Saturday gives Monday, and subtracting one from a Sunday gives Friday.

Will daylight saving change results?

No. Only calendar dates are used, with no clock times, so DST cannot move a result by a day.

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