using linq in C# to get DateTime range from list

I am using linq(in c#) to get the records going back 1 year back. My code below works if i use .AddMonths(-10);  but if I used AddMonths(-12); all the records are returned. I'm not sure why this occurs.  

Could someone tell how can i get the previous 12 month using linq.  




                  var result = items
                    .Where(x => !x.IsDeleted)
                    .OrderByDescending(x => x.OpenedAt)                  
                    .Take(perPage)
                    .ToList();




                var startDate1 = DateTime.Now.AddMonths(-10);              
                var newResult1 = result.Where(x => x.CreatedAt >= startDate1);


                var startDate2 = DateTime.Now.AddMonths(-12);              
                var newResult2 = result.Where(x => x.CreatedAt >= startDate2);