Checking to see if date is true

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C#

ASP.NET

Hi all, got a slight problem when checking to see if the date is a proper date, im checking to see if the date is no more than 31 days and also to check if the "To date" is greater than the "From Date" those 2 functions work but now i got to check if those dates are valid so you cant for example say "2012/10/45",

how can i do this my code for the date is as follows

{
        DateTime dt = Convert.ToDateTime(TextBox1.Text);
        DateTime dta = Convert.ToDateTime(TextBox2.Text);
        TimeSpan pp = dta - dt;
        int days = pp.Days;

        string s = TextBox1.Text;
        string t = TextBox2.Text;

        DateTime theDateA = DateTime.Parse(s);
        DateTime theDateB = DateTime.Parse(t);

        if (theDateB > theDateA)
        {
            if (days > 31)
            {
                TextBox1.BackColor = System.Drawing.Color.Red;
                TextBox2.BackColor = System.Drawing.Color.Red;
                date_Error_cell.Text = "Select a DATE NOT more than 31 days";
                date_Error_cell.Attributes.Add("style", "font-weight:bold; text-align:center; font-family:Calibri; color:Red; font-size:large;");
                Monthly_Spend();
                label();
            }
            else
            {

                TextBox1.BackColor = System.Drawing.Color.White;
                TextBox2.BackColor = System.Drawing.Color.White;
                date_Error_cell.Text = "";
                bind_grid();
                Monthly_Spend();
            }
        }
        else
        {
            date_Error_cell.Text = "Select a VALID date";
        }
    }