Spróbuj takiego zapisu, za pomocą klauzuli BETWEEN i pobrania tylko wskazanej daty z dateTimePicker-a
string from = dateTimePicker1.Value.ToShortDateString() + " 00:00",
to = dateTimePicker1.Value.ToShortDateString() + " 23:59";
string sql = "SELECT * FROM wartosci WHERE datetime1 BETWEEN '" + from + "' IN '" + to + "';";
lub
string from = dateTimePicker1.Value.ToShortDateString() + " 00:00",
to = dateTimePicker1.Value.AddDays(1).ToShortDateString() + " 00:00";
string sql = "SELECT * FROM wartosci WHERE datetime1 BETWEEN '" + from + "' IN '" + to + "';";