Extends System.DateTime to provide various utility functions.

DateTimeExtensions.cs

namespace System
{
    public static class DateTimeExtensions
    {
        /// <summary>
        /// Returns the Week Number of the supplied date object
        /// </summary>
        /// <param name="value"></param>
        /// <returns>int</returns>
        public static int WeekNumber(this System.DateTime value)
        {
            return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(value, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday);

        }

        /// <summary>
        /// Returns total number of weeks since 1/1/1970
        /// </summary>
        /// <param name="value"></param>
        /// <returns></returns>
        public static int EpochWeekNumber(this System.DateTime value)
        {
            TimeSpan t = value - new DateTime(1970, 1, 1);
            int secondsSinceEpoch = (int)t.TotalSeconds;
            int weeksSinceEpoch = secondsSinceEpoch / 604800;

            return weeksSinceEpoch;
        }

        /// <summary>
        /// Returns a DateTime object for the last day of the supplied dates week.
        /// </summary>
        /// <param name="dateTime"></param>
        /// <returns>Datetime</returns>
        public static DateTime EndOfWeek(this DateTime dateTime)
        {
            DateTime start = StartOfWeek(dateTime);

            return start.AddDays(7).AddSeconds(-1);
        }

        /// <summary>
        /// Returns a DateTime object for the first day of the supplied dates week.
        /// </summary>
        /// <param name="dateTime"></param>
        /// <returns>DateTime</returns>
        public static DateTime StartOfWeek(this DateTime dateTime)
        {
            int days = dateTime.DayOfWeek - DayOfWeek.Sunday;

            if (days < 0)
                days += 7;

            return dateTime.AddDays(-1 * days).Date;
        }

        /// <summary>
        /// Returns a DateTime object for the first day of the supplied dates preivous week.
        /// </summary>
        /// <param name="dateTime"></param>
        /// <returns></returns>
        public static DateTime StartOfPreviousWeek(this DateTime dateTime)
        {
            DateTime oneWeekAgo = dateTime.AddDays(-7);
            return FirstDateOfWeek(oneWeekAgo.Year, oneWeekAgo.WeekNumber());
        }

        /// <summary>
        /// Returns beginning date for a provided week of year
        /// </summary>
        /// <param name="year"></param>
        /// <param name="weekOfYear"></param>
        /// <returns></returns>
        public static DateTime FirstDateOfWeek(int year, int weekOfYear)
        {
            DateTime jan1 = new DateTime(year, 1, 1);
            int daysOffset = (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek - (int)jan1.DayOfWeek;

            DateTime firstMonday = jan1.AddDays(daysOffset);

            int firstWeek = jan1.WeekNumber();

            if (firstWeek <= 1)
            {
                weekOfYear -= 1;
            }

            return firstMonday.AddDays(weekOfYear * 7);
        }
    }
}