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Dates & Time · Date arithmetic & calendars

ISO Week Number & Week-Date Converter

Convert a Gregorian calendar date to ISO week-year, week number, and weekday, or convert a valid ISO week date back to a calendar date.

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

ISO week 1 is the week containing 4 January; ISO weekdays run Monday=1 through Sunday=7.

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

2021-01-01 belongs to ISO week 53 of ISO week-year 2020.

ISO week 1 is the week containing 4 January; ISO weekdays run Monday=1 through Sunday=7.

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

ISO week-year can differ from calendar year near January and December. Invalid week 53 combinations are rejected.

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