Calendar.ISOWeek.cs

namespace System.Globalization
{
    public partial class Calendar
    {
        public static class ISOWeek
        {
            private static int GetWeek(DateTime time)
            {
                var week = GetWeekNumber(time);

                if (week < 1)
                {
                    // The given date belongs to the preceding (week-based) year.
                    return GetWeeksInYear(time.Year - 1);
                }

                if (week > GetWeeksInYear(time.Year))
                {
                    // The date is actually in week 1 of the following year.
                    return 1;
                }

                return week;
            }

            private static int GetYear(DateTime time)
            {
                var week = GetWeekNumber(time);

                if (week < 1)
                {
                    // The given date belongs to the preceding (week-based) year.
                    return time.Year - 1;
                }

                if (week > GetWeeksInYear(time.Year))
                {
                    // The date is actually in week 1 of the following year.
                    return time.Year + 1;
                }

                return time.Year;
            }

            public static DateTime GetYearStart(int year)
            {
                // Gets Monday in the first week of the year.
                return GetDate(year, 1, DayOfWeek.Monday);
            }

            public static DateTime GetYearEnd(int year)
            {
                // Gets Sunday in the last week of the year.
                return GetDate(year, GetWeeksInYear(year), DayOfWeek.Sunday);
            }

            public static DateTime GetDate(int year, int week, DayOfWeek dayOfWeek)
            {
                var jan4DayOfWeek = new DateTime(year, 1, 4).DayOfWeek;

                var dayOfYear = (week * 7) + GetWeekday(dayOfWeek) - (GetWeekday(jan4DayOfWeek) + 3);

                return new DateTime(year, 1, 1).AddDays(dayOfYear - 1);
            }

            public static int GetWeeksInYear(int year)
            {
                int P(int y) => (y + (y / 4) - (y / 100) + (y / 400)) % 7;
                return P(year) == 4 || P(year - 1) == 3 ? 53 : 52;
            }

            private static int GetWeekNumber(DateTime time)
            {
                return (time.DayOfYear - GetWeekday(time.DayOfWeek) + 10) / 7;
            }

            private static int GetWeekday(DayOfWeek dayOfWeek)
            {
                return ((int) dayOfWeek + 6) % 7 + 1;
            }
        }
    }
}