System.Globalization.Calendar.GetFirstDayWeekOfYear C# (CSharp) Method

        internal int GetFirstDayWeekOfYear(DateTime time, int firstDayOfWeek) {
            int dayOfYear = GetDayOfYear(time) - 1;   // Make the day of year to be 0-based, so that 1/1 is day 0.
            // Calculate the day of week for the first day of the year.
            // dayOfWeek - (dayOfYear % 7) is the day of week for the first day of this year.  Note that
            // this value can be less than 0.  It's fine since we are making it positive again in calculating offset.
            int dayForJan1 = (int)GetDayOfWeek(time) - (dayOfYear % 7);
            int offset = (dayForJan1 - firstDayOfWeek + 14) % 7;
            BCLDebug.Assert(offset >= 0, "Calendar.GetFirstDayWeekOfYear(): offset >= 0");
            return ((dayOfYear + offset) / 7 + 1);
        }