DateTime Extension Method to Give Week Number

Introduction

Week number is quite complex: ISO 8601[^] specifies that weeks start on Monday, and that Jan 1^st is in week one if it occurs on a Thursday, Friday, Saturday, or Sunday. Otherwise it is in the last week of the previous year, which may be week 51, 52, or 53.

Over the years, I have come up with a couple of ways of handling this, but this version uses recursion to identify the last week of the previous year, and is pretty simple to understand.

public static class ExtensionMethods
    {
    /// <summary>
    /// Offsets to move the day of the year on a week, allowing
    /// for the current year Jan 1st day of week, and the Sun/Mon 
    /// week start difference between ISO 8601 and Microsoft
    /// </summary>
    private static int[] moveByDays = { 6, 7, 8, 9, 10, 4, 5 };
    /// <summary>
    /// Get the Week number of the year
    /// (In the range 1..53)
    /// This conforms to ISO 8601 specification for week number.
    /// </summary>
    /// <param name="date"></param>
    /// <returns>Week of year</returns>
    public static int WeekOfYear(this DateTime date)
        {
        DateTime startOfYear = new DateTime(date.Year, 1, 1);
        DateTime endOfYear = new DateTime(date.Year, 12, 31);
        // ISO 8601 weeks start with Monday 
        // The first week of a year includes the first Thursday 
        // This means that Jan 1st could be in week 51, 52, or 53 of the previous year...
        int numberDays = date.Subtract(startOfYear).Days + 
        				moveByDays[(int) startOfYear.DayOfWeek];
        int weekNumber = numberDays / 7;
        switch (weekNumber)
            {
            case 0:
                // Before start of first week of this year - in last week of previous year
                weekNumber = WeekOfYear(startOfYear.AddDays(-1));
                break;
            case 53:
                // In first week of next year.
                if (endOfYear.DayOfWeek < DayOfWeek.Thursday)
                    {
                    weekNumber = 1;
                    }
                break;
            }
        return weekNumber;
        }
    }

It works by calculating the number of days since the start of the year, and moving that forward by a week to get a one-based answer. This is offset back a week, if the start of the year is a Monday, Tuesday or Wednesday, and adjusted for the DateTime.DayOfWeek return of Sunday as the week start - hence the odd numbering of the moveByDays array elements.