Primary source for current and historical civil time-zone rules.
Date & time
Week-number calculator
Enter your values below for a clear, instant result.
Your result
ISO week
FormulaISO 8601: week starts Monday and week 1 contains January 4.
- Enter the known values.
- Apply the displayed formula.
- Review the result and supporting values.
Worked example: The ISO week year can differ near New Year.
Accuracy notice: The arithmetic follows the displayed formula. Its practical accuracy still depends on valid inputs, appropriate units, and display rounding.
Understand the result before using it
Week-number calculator uses date to calculate its result. It evaluates calendar or clock values using the date, duration, and time-zone rules represented by the tool.
Formula, assumptions, and example
Formula
ISO 8601: week starts Monday and week 1 contains January 4.
Assumptions
- The entered date are complete, valid, and use the units or format shown beside each field.
- Entered dates use the Gregorian calendar unless the tool explicitly states another system.
- The device and time-zone database contain the rules needed for the selected date.
Worked example
The ISO week year can differ near New Year.
When to use it
Good uses
- Compare dates, durations, work periods, or timestamps.
- Plan an event or deadline across calendars or time zones.
- Check a date calculation before adding it to a schedule.
Limitations
Common mistakes to avoid
- Governments can change daylight-saving and civil time-zone rules.
- Business days and holidays depend on the organization and jurisdiction; confirm the applicable calendar.
- Check the formula, source data, units, and displayed precision before relying on the result.
Sources and further reading
These references explain the underlying standards or provide authoritative context. Your own contract, institution, clinician, product documentation, local code, or governing standard may be the controlling source.
Reference information for timekeeping and time standards.
Questions about this calculation
What does the Week-number calculator calculate?
Week-number calculator uses date to calculate its result. It evaluates calendar or clock values using the date, duration, and time-zone rules represented by the tool. The formula used is: ISO 8601: week starts Monday and week 1 contains January 4.
Which inputs affect the Week-number calculator result?
The result uses date. Changing any of these values can change the output, so enter them using the labels and units shown.
Is the Week-number calculator result exact?
The arithmetic follows the displayed formula. Its practical accuracy still depends on valid inputs, appropriate units, and display rounding.
When should I verify a Week-number calculator result?
Verify it whenever the source measurement, unit convention, rounding, or consequences make small differences important.
Clear answers, without the guesswork
1Enter your information
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2Review the calculation
The formula and context explain how the result was reached.
3Save or share it
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