How Can I Accurately Calculate the Start Date of a Given Week Number Using the ISO 8601 Standard?

Determining the First Day of a Week Based on ISO 8601 Week Number

This improved algorithm calculates the start date (Monday) of a given ISO 8601 week number and year. Previous approaches had inconsistencies, particularly with edge cases like week 53. This solution leverages the ISO 8601 standard's definition of the week (starting on Monday and the first week containing a Thursday).

The process begins by establishing the first day of the specified year (January 1st). The day-of-week offset is then calculated to reach the nearest Thursday. This Thursday is crucial because it anchors the first week of the year according to ISO 8601.

The algorithm next determines the first Thursday of the year and, based on ISO 8601 rules, identifies the first week. The provided week number is then adjusted to account for this Thursday-based week numbering.

Finally, the correct number of days is added to the initial Thursday to arrive at the first day of the target week. To adhere strictly to ISO 8601, three days are subtracted from this date to obtain the Monday that marks the beginning of the week.

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