Free Online Date Calculator

A date calculator adds or subtracts years, months, weeks and days from a starting date, respecting leap-year rules and ISO 8601 week numbering on the result.

Your data is processed entirely in your browser and never sent to any server.

Instant results 100% private No signup needed
Resulting date
May 26, 2026
Tuesday · ISO 2026-05-26
Day of year
146
ISO week number
22

How to Use This Tool

  1. Pick a start date with the date picker.
  2. Choose Add or Subtract.
  3. Enter years, months, weeks and days to apply (any mix; leave others at 0).
  4. Read the resulting date, weekday, day-of-year and ISO week number below.

What Is a Date Calculator?

Calendar arithmetic gets awkward in the corners: adding a month to January 31 has to pin to the end of February, and adding a year to February 29 has to handle the missing leap day. A date calculator does that bookkeeping for you. Internally it uses the browser's native Date object, which follows ISO 8601 conventions for week numbering — week 1 is the one containing the first Thursday of the year, with weeks starting on Monday.

Project managers use it to set milestone dates from a kickoff. Legal teams compute statute-of-limitations windows. Anyone planning international travel works out the return date from a departure and a trip length. The result panel shows the new date in your locale, an ISO 8601 string for copying into spreadsheets, the weekday name, the day-of-year (1–366) and the ISO week number — useful when scheduling alongside payroll, sprint or fiscal-week calendars.

Everything runs locally using the native Date object; no server round-trip, nothing logged.

Frequently Asked Questions

What can I calculate with this tool?
Pick a starting date and add or subtract any mix of years, months, weeks and days. The result shows the new date, what day of the week it falls on, day-of-year and ISO week number.
Does it handle leap years correctly?
Yes. Year and month arithmetic uses JavaScript's native Date object, which respects leap years (Feb 29 only exists in leap years, Feb + 1 month from Jan 31 lands on the last day of February).
How are business days handled?
This tool counts calendar days. If you need to skip weekends and holidays, use the Work Days Calculator instead.

Published by the WeGotEveryTool team. We build and test every tool in-house and update pages when the underlying spec, formula, or recommendation changes.

Reviewed: May 2026. Disclaimer: this tool is provided as-is for general informational use. For decisions with material consequences (medical, legal, financial, security) verify results against a qualified professional source.

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Work Days Calculator

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