public String getSecondsString() {
            if (timeUnits == HH_MM_SS) return input.substring(4, 6);
            else if (timeUnits == MM_SS) return input.substring(2, 4);
            else return "00";
        }
Encapsulates the digit input logic and text to duration conversion logic.
android/third_party/time-duration-picker/src/main/java/mobi/upod/timedurationpicker/TimeDurationPicker.java