How to Accurately Calculate the First Day of a Week (Monday) Given its Year and Week Number?

Determining the First Day of a Week (Monday) Using C#

This article presents a refined method for calculating the date of the first day of a given week (Monday) in C#, using the year and week number as input. The original approach had limitations due to its reliance on Monday as the starting point.

Leveraging the ISO 8601 Standard

The ISO 8601 standard provides a more robust approach by using Thursday as the reference day for week numbering. This method adheres to this standard for greater accuracy and consistency.

First Thursday and Week Number Adjustment

The algorithm begins by identifying the first Thursday of the year. The week number is then adjusted to correctly align with ISO 8601 definitions.

The Calculation Process

The first day of the target week is calculated by adding seven days for each week number to the initial first Thursday. Subsequently, three days are subtracted to obtain the Monday of that week, consistent with the ISO 8601 week numbering system.

Ensuring Accurate Results

By following the ISO 8601 standard and using the first Thursday as the anchor, this method guarantees a precise calculation of the first day of any given week based on its year and week number.

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