Don't do it like that! Why are you taking a string, converting it to a DateTime, then back to a string again, to add to another string, to pass to SQL where it has to be converted back to a DateTime to compare it? Use parametrized queries instead (You should be anyway to avoid SQL injection attacks)
string vFDate, vTDate, vStr;
DateTime vFDate = Convert.ToDateTime(txtFDate.Text);
DateTime vTDate = Convert.ToDateTime(txtTDate.Text);
vStr = "SELECT * FROM tbl WHERE R_Date BETWEEN @FD AND @TD";
OleDbCommand cmd = new OleDbCommand(vStr, OleDBComm);
cmd.Parameters.AddWithValue ("@FD", vFDate);
cmd.Parameters.AddWithValue ("@TD", vTDate);
OleDBConn.Open();
cmd.ExecuteNonQuery();
Chances are, you problem will disappear at the same time!
But if you must use a text box for date entry (and I'd use a DateTimePicker instead), then you should use DateTime.TryParse instead of Convert.ToDateTime.