How to create month columns in database based on number of months and start date, end date

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C#

ASP.NET

SQL-Server-2008R2

i have No of months and Start date and end date in my page


when i give number of months 3 and start date 1/6/2015 then end date will calculated to 1/9/2015 automatically

i want to create extra columns in database i.e June-2015, july and august
in this table or another table

my code is

protected void Button1_Click(object sender, EventArgs e)
   {
       SqlCommand cmd = new SqlCommand("Insert into  monthcal (NoOfMonths,StartDate,EndDate) values (@NoOfMonths,@StartDate,@EndDate)", con);
       cmd.Parameters.AddWithValue("@NoOfMonths", txtNoOfMonths.Text.ToString());
       cmd.Parameters.AddWithValue("@StartDate", txtStartDate.Text.ToString());
       cmd.Parameters.AddWithValue("@EndDate", txtEndDate.Text.ToString());
       con.Open();
       cmd.ExecuteNonQuery();

   }
   protected void txtStartDate_TextChanged(object sender, EventArgs e)
   {
       string inputString = txtStartDate.Text;
       DateTime dt = DateTime.ParseExact(inputString, "yyyy/MM/dd", CultureInfo.InvariantCulture);
       dt = dt.AddMonths(Convert.ToInt32(txtNoOfMonths.Text));
       txtEndDate.Text = dt.ToString("yyyy/MM/dd");


       DateTime Date1 = Convert.ToDateTime(txtStartDate.Text);
       DateTime Date2 = Convert.ToDateTime(txtEndDate.Text);
       //int Datediff = ((Date2.Year - Date1.Year) * 12) + Date1.Month - Date2.Month;
       int DayDiff = (Date2.Date - Date1.Date).Days;
       Label1.Text = "Total days" + " " + (DayDiff.ToString());