Try this:
DateTime doj = new DateTime();
System.Globalization.CultureInfo c1 = new System.Globalization.CultureInfo("en-GB", true);
doj = DateTime.Parse(txtdob.Text.Trim(), c2, System.Globalization.DateTimeStyles.NoCurrentDateDefault);
and insert like this
insert into table (dob) values ('" + doj.ToShortDateString()+ "')