This contains #Extension methods for #DateTime objects. The method WeekSpan gives you Tuple<DateTime,DateTime? representing a week from the start of the day a week ago to the end of the DateTime instance calling the method.

DateTimeExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MyExtensions
{
    
    public static class DateTimeExtensions
    {
        /// <summary>
        /// Returns a tuple representing a week span (StartOfWeek,EndOfWeek), given a particular date (which is the last day of the week).
        /// For example, given Tuesday 12:01pm, will return (PrevWednesday 00:00:00:000, Tuesday 23:59:59:999)
        /// </summary>
        /// <param name="endOfWeek"></param>
        /// <returns></returns>
        public static Tuple<DateTime, DateTime> WeekSpan(this DateTime endOfWeek)
        {
            DateTime weekEnded = new DateTime(endOfWeek.Year, endOfWeek.Month, endOfWeek.Day, 23, 59, 59,999);
            DateTime weekStarted = weekEnded.AddDays(-7).AddMilliseconds(1);
            return new Tuple<DateTime, DateTime>(weekStarted, weekEnded);
        }


        public static DateTime StartOfDay(this DateTime theDateTime)
        {
            return new DateTime(theDateTime.Year, theDateTime.Month, theDateTime.Day, 0, 0, 0, 0);
        }

        public static DateTime EndOfDay(this DateTime theDateTime)
        {
            return new DateTime(theDateTime.Year, theDateTime.Month, theDateTime.Day, 23, 59, 59, 999);
        }
    }
}