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ISO Week Date Converter
Convert strict calendar dates and ISO week dates using Monday-based weeks, with full week boundaries in the download.
Strict YYYY-MM-DD, Strict YYYY-Www-D or week-only YYYY-Www
JSON ISO week-date report
Browser only
Runs locally in your browser. Your files, pasted content and generated output are not uploaded. Reset clears this tool’s working data.
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How to use this tool
- Choose Calendar to ISO week or ISO week to calendar.
- Enter the strict matching date form; week-only ISO input defaults to Monday.
- Select Convert week date and download iso-week-date.json.
Examples
Calendar date 2021-01-01 is ISO 2020-W53-5, in the week 2020-12-28 through 2021-01-03.
Limitations
- Years 0001 through 9999, 32 raw input UTF-16 units, 64 KiB report and 30 seconds.
- Nonexistent ISO week 53 and any result or complete week boundary outside the supported year domain are rejected. No local timezone or current-clock dependency.
- No locale/fiscal week rules, holidays or unrestricted historical calendar claims.
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