CALCULATE WITH CONFIDENCE

Dates & Time · Date arithmetic & calendars

Date Add, Subtract & Missing Date Calculator

Add or subtract calendar years, months, weeks, and days, or solve a missing start date using a declared end-of-month convention.

Dates & Time · Date arithmetic & calendars

Enter the date and time scenario

Private in-browser workflow · no account, upload, location tracking, or saved schedule

Enter the calendar, clock, named-zone, workweek, and closure assumptions that actually govern the task. A named IANA zone models offset transitions; a fixed abbreviation such as CST is intentionally not accepted. Nothing entered here is added to a URL, analytics event, account, or saved schedule.

Date and time result

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Enter valid values to see the result.

Your entries are calculated in this browser and are not submitted to 365CALCS.COM.

Quick guide

How to use this calculator

  1. Choose the exact calendar, clock, recurrence, time-zone, working-day, or endpoint convention before entering values.
  2. Add one structured row for each participant, interval, stage, closure, target, or period that belongs in the scenario.
  3. Check the component ledger before using the headline result: it exposes skipped dates, offsets, transitions, overlaps, stage boundaries, and the convention driving the answer.

Calculation method

Explicit date and time workflow

Calendar years and months are applied first with the selected invalid-day rule, followed by exact seven-day weeks and calendar days.

The calculator keeps civil dates, elapsed durations, local clock readings, UTC instants, work calendars, and recurring rules distinct rather than silently treating them as interchangeable.

Worked example

Worked example

Adding one month to 2026-01-31 clamps to 2026-02-28 under the last-valid-day convention.

Calendar years and months are applied first with the selected invalid-day rule, followed by exact seven-day weeks and calendar days.

Supported inputs

Precision and limits

Calendar and time-zone conventions

Gregorian dates and IANA named zones are modeled explicitly. Holiday, school, legal, payroll, contractual, and operating calendars are never inferred from a country name.

DST and civil-time limits

Repeated or nonexistent local clock readings are identified where they matter. Future government time-zone changes cannot be predicted beyond the time-zone data available to the runtime.

Planning support, not authority

Results organize entered assumptions. They do not determine legal notice, payroll compliance, staffing safety, entitlement, service credits, travel feasibility, or institutional policy.

Calculator-specific boundary

Reject mode reports impossible dates; clamp mode uses the last valid day; preserve-end mode keeps month-end dates at month end.

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