This is the solution. Try this
string filter = " (Date >= #" +
Convert.ToDateTime(txtStartDate.Text).ToString("MM/dd/yyyy") +
"# And Date <= #" +
Convert.ToDateTime(txtEndDate.Text).ToString("MM/dd/yyyy") +
"# ) ";
DataView dvFormula = dsFormula.Tables[0].DefaultView;
dvFormula.RowFilter = filter ;