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Add or subtract days and diff dates. Use for durations; for workdays use business-days-calculator. Input: dates. Returns difference. 100% client-side, files never leave browser.
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Performs date arithmetic and computes the exact duration between two dates. Add or subtract days, weeks, months, or years from a start date, and view the resulting date plus a day-of-week calendar preview. Also calculates the precise difference (years, months, days, hours, minutes) between any two dates.
Date math is deceptively tricky — months and years have uneven lengths, and adding 'one month' can land on an invalid day. A dedicated calculator removes the ambiguity and produces correct calendar arithmetic every time.
This tool is part of the DevStackIO platform — a collection of free online developer tools from DevStackIO. Browse more free developer resources on DevStackIO.
Project managers planning milestones, developers computing deadlines and durations, HR teams handling leave and tenure, and anyone tracking event or payment schedules.
Choose the start date using the date picker
Select Add or Subtract and enter an amount in days, weeks, months, or years
Read the resulting date and its day of the week
For a difference, pick two dates and view the span in years, months, days, and time
Copy the result for use in schedules or documentation
Start 2026-08-07, add 90 days → 2026-11-05 (a Thursday)Start 2026-01-31, add 1 month → 2026-03-03 (clamped to February's end)Difference 2025-01-01 to 2026-08-07 → 1 year, 7 months, 6 daysStart 2026-08-07, subtract 6 weeks → 2026-06-26How does adding a month handle month-end?
Adding one month to January 31 lands on February 28 (or 29), since the target day is clamped to the last valid day.
Is this intentional-shift arithmetic?
Yes, it follows natural calendar addition rather than elapsed-time arithmetic.
Does it handle leap years?
Yes, February dates account for leap years automatically.
Is anything sent to a server?
No, all computations run locally in your browser.
Authoritative specifications, standards, and in-depth reading for Date Calculator.
Result
November 11, 2026
Wednesday · +30 days from October 12, 2026
Month-end dates clamp to the last valid day (e.g., Jan 31 + 1 month = Feb 28). Leap years are handled. Runs locally.
Calculate the exact number of days, weeks, months, and years between two dates. Handles leap years and varying month lengths correctly.
Calculate exact age from birthdate. Use for years/months/days; for differences use date-calculator. Input: date. Returns age. 100% client-side, files never leave browser.
Count working days (Mon–Fri) between two dates, or add/subtract business days. Leap-year and month-length correct.
Format dates into ISO, RFC, and locale strings. Use for display formats; for Unix epochs use timestamp-converter. Input: date text. Returns formatted dates. 100% client-side.
Find the day of week for any date. Shows weekday name, day-of-year, and leap year status. Uses proleptic Gregorian calendar.
Find the ISO 8601 week number, weekday, day-of-year, and leap-year status for any date.
Count characters, words, lines, and bytes. Use for quick length checks; for full stats use word-counter. Input: text ≤10MB. Returns counts. 100% client-side.
Compute similarity % and Levenshtein distance between strings. Use for fuzzy match scoring; for visual diff use text-diff-visual. Input: two strings ≤10MB. Returns similarity + distance. 100% client-side.