Human Readable Duration

DurationPattern.java

long now = System.currentTimeMillis();
doSomeLongTask();
long duration = System.currentTimeMillis() - now;

HumanReadableUtils.java

package com.example.commons.humanreadable.utils;

import java.util.Arrays;
import java.util.List;
import java.util.concurrent.TimeUnit;

import org.apache.commons.lang.WordUtils;

public final class HumanReadableUtils {
    private static final List<TimeUnit> timeUnits = Arrays.asList(TimeUnit.DAYS, TimeUnit.HOURS, TimeUnit.MINUTES,
            TimeUnit.SECONDS);

    public static String toHumanReadableDuration(final long millis) {
        final StringBuilder builder = new StringBuilder();
        long acc = millis;
        for (final TimeUnit timeUnit : timeUnits) {
            final long convert = timeUnit.convert(acc, TimeUnit.MILLISECONDS);
            if (convert > 0) {
                builder.append(convert).append(' ').append(WordUtils.capitalizeFully(timeUnit.name())).append(", ");
                acc -= TimeUnit.MILLISECONDS.convert(convert, timeUnit);
            }
        }
        return builder.substring(0, builder.length() - 2);
    }
}

It crashes with null exception at line 23 if for example are millis is passed 231 ms.