How to use this developer tool
- Paste or type a value in the input area.
- Choose the conversion option when one is available.
- Select Convert ISO Week Date or press Ctrl/Cmd + Enter.
- Review the interpretation and copy or download the result.
ISO Week Date Converter
Convert Gregorian calendar dates to ISO week dates and reverse ISO week notation back to a calendar day. ISO weeks start on Monday, and week 1 is the week containing January 4. Those rules mean the ISO week-year can differ from the calendar year near New Year, so the output includes both values and the Monday-to-Sunday range for the selected week.
Developer Input
Calendar date or ISO week dateAd space
What this developer tool does
An ISO week date contains a week-year, a two-digit week number, and weekday 1 through 7 for Monday through Sunday.
Anchor the calculation on Thursday to determine the ISO week-year, then count seven-day intervals from the first ISO Thursday.
The converter uses UTC calendar arithmetic, avoiding daylight-saving transitions while moving between days.
Example
Try the sample input, then run the converter. The output panel reports the normalized value and the fields used to derive it.
2026-07-19
Use cases
- Inspect values copied from logs, headers, or API payloads.
- Normalize configuration before committing it.
- Compare timestamp representations during debugging.
- Create a reproducible test fixture for date-handling code.
Tips for reliable output
- Include an explicit UTC marker or numeric offset for portable instants.
- Keep the original input beside a converted value during debugging.
- Test boundary dates near midnight and New Year.
- Do not assume a fixed offset carries daylight-saving rules.
- Confirm the consumer uses the same token or date standard.
Processing details
Anchor the calculation on Thursday to determine the ISO week-year, then count seven-day intervals from the first ISO Thursday.
The converter uses UTC calendar arithmetic, avoiding daylight-saving transitions while moving between days.
Frequently asked questions
What input does the ISO Week Date Converter accept?
Use the format shown in the sample. The parser rejects empty input and reports malformed values instead of guessing silently.
Does the ISO Week Date Converter use my local timezone?
UTC and offset-aware values represent a fixed instant. Any output labeled local uses the timezone configured in your browser.
Can I use the output in production code?
The output is copy-ready, but verify it against the date library and runtime used by your application, especially for legacy formats.
Why can date results differ between browsers?
Browsers may ship different timezone database versions and may parse non-standard legacy date strings differently. Prefer explicit standard formats.
Is my date or duration uploaded to a server?
No. The conversion runs in this page with browser APIs and does not require a network request.
Related developer tools
Time format reference
| Value | Meaning |
|---|---|
| UTC | Coordinated Universal Time |
| Unix epoch | 1970-01-01T00:00:00Z |
| ISO 8601 | International date and time notation |