human_duration.js

// requires jQuery

function humanDuration(milliseconds) {
  var remainder = milliseconds;

  var times = [
    (remainder /= 1000) % 60,  // seconds
    (remainder /= 60)   % 60,  // minutes
    (remainder /= 60)   % 24,  // hours
    (remainder /= 24),         // days
  ].reverse();

  // zero pad to 2 digits
  var padded = jQuery.map(times, zeroPad(2));

  // compress all leading zero digits
  return padded.join(':').replace(/^(?:00:)+/, '00:');
}

function zeroPad(padTo) {
  return function(number) {
    var padded = Math.floor(number).toString();

    while (padded.length < padTo)
      padded = '0' + padded

    return padded;
  };
}

human_duration.rb

def human_duration(milliseconds)
  remainder = milliseconds.to_i
  time = [
    (remainder /= 1000) % 60,  # seconds
    (remainder /= 60)   % 60,  # minutes
    (remainder /= 60)   % 24,  # hours
    (remainder /= 24),         # days
  ].reverse
  ('%02d:%02d:%02d:%02d' % time).sub(/\A(?:00:)+/, '00:')
end