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CalcSpan/Universal date and time/Add or Subtract Date Calculator
P0 · Date arithmeticNo hidden counting rules.

Add or Subtract Date Calculator

Move a date forward or backward with the unit and boundary rule kept visible—especially where month ends and business-day conventions can change the answer.

Universal engineAdd or Subtract Date CalculatorFree · local-first · explicit assumptions
Reference calculation

Enter your details.

Calculated locally in your browser. Nothing is uploaded to CalcSpan.

Switch modes without leaving the page. Each result states its counting convention.
Sun, Aug 23, 2026

Plain-date semantics · start date is not counted unless the result explicitly says inclusive.

Your result
Sep 22, 2026 · Tuesday

Aug 23, 2026 + 30 days.

Starting dateAug 23, 2026 · Sunday
OperationAdd 30 days
Result dateSep 22, 2026 · Tuesday
Calendar distance30 daysThe number of date boundaries crossed.
Equivalent span4 weeks + 2 daysStandard date addition treats the starting date as the reference boundary, not as day 1.
Business-day start settingBegin after the start dateSaved for Business days; not applied to the selected unit.
Month-end settingClamp to the last valid daySaved for month/year operations; not applied to the selected unit.
Business-day calendar settingSaturday and Sunday · U.S. federal holidaysSaved for Business days; not applied to the selected unit.
Timeline
Sep 22, 2026Added 30 days from Aug 23, 2026
Calendar options

Add to calendar

Choose a calendar service, then confirm the event. Downloading an .ics file does not add it until you open or import it.

Read before relying

Context matters.

  • Calendar-day, week, month and year operations use date-only Gregorian arithmetic; daylight-saving changes do not alter the result.
  • Month-end policy is ignored unless the selected unit is months or years.
Rule set 2.0.0

Sources & scope

Source & calculation QA reviewed

The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.

Ecma International / TC39 · SRC-ECMA402-DATETIME-001ECMA-402 Internationalization API — DateTimeFormat and IANA time-zone identifiersSource retrieved 2026-08-22Internet Assigned Numbers Authority · SRC-IANA-TZDB-001Time Zone Database — civil-time history and daylight-saving rule updatesSource retrieved 2026-08-22U.S. Office of Personnel Management · SRC-OPM-FEDERAL-HOLIDAYS-001Federal Holidays — work schedules, observance and paySource retrieved 2026-08-22
JurisdictionGlobal Gregorian date arithmetic and IANA time zones; optional U.S. federal holiday planning calendar
Last reviewed2026-08-22
Next review due2027-02-22
Review scopeCalcSpan source & calculation QA
PrivacyNo input upload
How it works

Date arithmetic with the edge cases exposed.

Calendar days and weeks move by fixed date boundaries. Months and years need a month-end policy. Business days need weekend, holiday and start-counting choices.

  1. 01Choose a starting date

    This remains the anchor for the operation and is shown beside the result.

  2. 02Set direction, amount and unit

    Add or subtract up to 1,000 calendar days, weeks, months, years or business days.

  3. 03Confirm the edge-case rule

    Review month-end clamping or the selected weekend, holiday and start-date conventions.

Days and weeks

Calendar days cross one date boundary each; a week always advances exactly seven calendar days.

Months and years

If the original day is missing in the target month, clamp uses that month's last valid day. Preserve-end-of-month keeps an end-of-month anchor at month end.

Business-day operations

The default excludes Saturday, Sunday and observed U.S. federal holidays, beginning after the starting date.

Rule trail

Rule set 2.0.0. Last reviewed 2026-08-22. Next review due 2027-02-22.

Questions answered

What to know before using the result.

What date is 30 days from today?

Choose today as the starting date, Add, 30 and Calendar days. The result crosses 30 date boundaries and does not count today as day one.

What happens when I add a month to January 31?

With the default clamp rule, the result is the last valid day of February. Preserve end of month also keeps month-end dates aligned in later months.

Can I subtract business days?

Yes. Choose Subtract and Business days, then set the weekend pattern, holiday option and whether an eligible starting date counts.

Does one week always mean seven days?

Yes in this tool. Weeks are fixed seven-calendar-day steps; they are not the same as five business days.

Use the same engine

Related date calculations.

Date CalculatorUse one transparent engine to measure a date span, move a date forward or backward, count business days, or compare local date-times across time zones.Days Between Dates CalculatorEnter any two dates to see the total day span, calendar year-month-day breakdown, full weeks, weekdays and weekend dates under an explicit counting rule.Business Days CalculatorCount working days in a date range without hiding the choices that matter: endpoint inclusion, weekend pattern and whether observed U.S. federal holidays are removed.Date and Time CalculatorCompare two local date-times across the same or different time zones, with daylight-saving gaps, repeated clock times and the difference between elapsed and wall-clock time made explicit.