DateTime Exception in LinQ query

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C#

LINQ

I have the follwing problem in my ASP project :
i use LinQ to Retrive data including date however it throw an exception
the Code:

DateTime? castvar = DateTime.Parse(TextBox1.Text);
       DateTime neededDay;
        //and then check it to give the value of the neededadte for linq query
       if (castvar != null)
       {

           neededDay = castvar.Value;


       }
       else
       {
           neededDay = DateTime.Now;
       }

            var DayReservations = (from r in db.reservations
                                  join p in db.Clints on r.clintId equals p.ClintID
                                  join d in db.Emplyees on r.doctorID equals d.EmpID
                                  where r.apointDateStart.Date == neededDay.Date
                                 && r.apointDateStart.Month == neededDay.Month
                                  && r.apointDateStart.Day == neededDay.Day
                                  && r.doctorID == Convert.ToInt32(DoctorList.SelectedValue)
                                  select new { p.Name, d.EmpName, r.apointDateStart, r.apointEndTime, r.suggestedProcedure, r.comment }).ToList();

the Exception :

Input string was not in a correct format.When converting a string to DateTime ,parse the string to take the date before putting each variable into the DateTime object.Make sure your method arguments are in right format.
this exception was thrown in the LinQ query



In the SQL the Reservation table is:

[clintId] = <clintId, int,>
     ,[doctorID] = <doctorID, int,>
     ,[apointDateStart] = <apointDateStart, datetime,>
     ,[apointDateEnd] = <apointDateEnd, datetime,>
     ,[comment] = <comment, nvarchar(max),>
     ,[suggestedProcedure] = <suggestedProcedure, nvarchar(max),>

where is the problem?????!!!!