Textbox Date does not show properly?

I have a textbox date:
<asp:textbox id="ttdate" Runat="server"></asp:textbox>
<input id="btnttDate" onclick="show_cele_date(btnttDate,'','',ttdate)"

When I see it's value in C# code
ttdate.Text is null and it is today date when I insert to DB

string StartDateTime = ttdate.Text.Trim() + " " + hour.SelectedItem.Value + ":" + minute.SelectedItem.Value + ":" + "00";
System.DateTime DT = Convert.ToDateTime(this.StartDateTime.ToString().Trim());

How can I get the selected date properly and format a datetime inserting into DB?
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