FirstDayOfWeek

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        /// <summary>
        /// Resolves the first day of the week of the given DateTime value.
        /// </summary>
        /// <param name="source">The given DateTimeValue.</param>
        /// <returns>The DateTime.Date of the first day of the week of the given DateTime value.</returns>
        /// <remarks>If the given DateTime is the first day of the week, then the source.Date will be the result.</remarks>
        public static DateTime FirstDayOfWeek(this DateTime source)
        {
            DayOfWeek day =  source.DayOfWeek;
            DayOfWeek weekBegin = System.Globalization.DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek;
            DateTime result;

            if (day >= weekBegin)
            {
                int offSet = day - weekBegin;


                result = source.AddDays(offSet * -1).Date;
            }
            else
            {
                result = source.AddDays(((int)day)-6);
            }

            return result;
        }