How Can I Reliably Calculate the First Day of a Week Given Its Year and Number in C#?

Determining the First Day of a Week Given Year and Week Number in C#

A frequent programming challenge involves calculating the date of a week's first day, given its year and week number. This is particularly important for European users who consider Monday the start of the week. A robust solution is crucial for accurate results.

A Reliable Method Using ISO 8601

The most dependable approach leverages the ISO 8601 standard. Unlike methods starting with Monday, ISO 8601 uses the year's first Thursday as its reference. This eliminates inconsistencies, especially when the first week of the year falls in the previous calendar year.

The following C# code snippet demonstrates this:

<code class="language-csharp">public static DateTime FirstDateOfWeekISO8601(int year, int weekOfYear)
{
    // ...
}</code>

This function starts with January 1st, calculates the day offset to the nearest Thursday, and uses this to pinpoint the year's first Thursday. It then applies the "FirstFourDayWeek" rule to determine the first week.

Subsequent weeks are calculated by adding (week number * 7) days to the first Thursday's date. Finally, the result is adjusted to Monday by subtracting 3 days.

This method provides a reliable way to calculate the first day of any week within a given year, handling various scenarios accurately.

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