Victor's

+        int hours, minutes, seconds;
+        long difference = endRentTime.toMillis(false) - rentTime.toMillis(false);
+        hours = (int) (difference / (1000 * 60 * 60));
+        minutes = (int) (difference - (1000 * 60 * 60 * hours)) / (1000 * 60);
+
+
+        costField.setText(String.format("%.0f %s", cost * difference / (1000 * 60), "р."));
+        timeField.setText(String.format("%d %s %d %s", hours, HOUR_FORMS[getRussianForm(hours)], minutes, MINUTE_FORMS[getRussianForm(minutes)]));
Такую логику нужно выносить в отдельный метод.