If a user doesn't fill in a date textbox on my ASP.NET form, I would like to put a blank date or a null into the dataset when the user clicks save. This is my current code

dsClinserv.Tables["cresult"].Rows[0]["date_in"] = Convert.ToDateTime(this.txtStart.Text).Date;

If tried dsClinserv.Tables["cresult"].Rows[0]["date_in"] = null; but this gives me an error.