Ajax calendar control selected date error converting string to system.datetime

cannot convert from 'System.Web.UI.WebControls.TextBox' to 'System.DateTime'

aspx:

<asp:ToolkitScriptManager ID="ToolkitScriptManager2" runat="server"> </asp:ToolkitScriptManager>  
                <asp:TextBox ID="txtFixedDate" runat="server"></asp:TextBox>
                <asp:CalendarExtender ID="CalendarExtender2" TargetControlID="txtFixedDate" Format="dd/MM/yyyy" runat="server" />

aspx.cs

protected void Page_Load(object sender, EventArgs e)
    {   txtFixedDate.Attributes.Add("readonly", "readonly");
       // txtFixedDate.Text = System.DateTime.Now.ToShortDateString();              
    }

protected void bnSubmit_Click(object sender, EventArgs e)
    {
       table db = new tableDb();
       table t = new table(

                       txtFixedDate.text          
           );

        db.Addtable(t);
    }

with txtFixedDate.text I am getting Error cannot convert from 'string' to 'System.DateTime'

In DB its a varchar field.