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CSharp · Console

/Program.cs

using System;
					
public class Program
{
	public static void Main()
	{
		var sunday = new DateTime(2019,11,10); // This is a Sunday
		var startOfTheWeek = sunday.AddDays(DayOfWeek.Monday - sunday.DayOfWeek); 

		Console.WriteLine("Date: " + startOfTheWeek); // This should give Monday 4th November 2019 but gives Monday 11th November 2019
	}
}