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ISO Week Number Calculation:
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The ISO week date system is part of the ISO 8601 date and time standard. It is used mainly in government and business for fiscal years and timekeeping. The system specifies a week year (also called ISO year) with 52 or 53 full weeks.
The ISO week-numbering year starts at the first day (Monday) of week 01 and ends at the Sunday before the new ISO year (or equivalently - week 01 of the next ISO year).
Details: Week numbers are widely used in business for project planning, reporting, and scheduling. Many European countries use week numbers in official documents and business communication.
Tips: Simply select any date from the date picker and click "Calculate" to find its ISO week number. The calculator will show both the week number and the corresponding ISO year.
Q1: Why does week 1 sometimes start in December?
A: Because week 1 is defined as the week containing the year's first Thursday, it may start in December if January 1st is a Friday, Saturday, or Sunday.
Q2: How many weeks are there in a year?
A: Most years have 52 weeks, but years where January 1st is a Thursday (or a Wednesday in leap years) have 53 weeks.
Q3: What's the difference between ISO week and US week numbering?
A: In the US system, weeks start on Sunday, and week 1 always contains January 1st. The ISO system is more consistent for business purposes.
Q4: Can week numbers be used for fiscal years?
A: Yes, many companies use week numbers for fiscal year planning, especially those operating internationally.
Q5: How do I convert week numbers back to dates?
A: You can use the formula: First day of week = January 1st + (week number - 1) * 7 - weekday of January 1st (adjusted for ISO rules).