/* Return a delta object whose days, hours, minutes, seconds property add up to
 * the amount of time between timeA and timeB. Assumes A happens before B. */
var duration = function (timeA, timeB) {
  var msPer = // milliseconds per various times
    { second: 1000
    , minute: 1000 * 60
    , hour:   1000 * 60 * 60
    , day:    1000 * 60 * 60 * 24
    };
  var delta = {};
  // number of milliseconds still unaccounted for by the delta object
  var ms = timeB - timeA;
  
  // if `ms` is negative, we make it positive so that the modulus/division
  // logic still works correctly. `one` is used again when saving the deltas so
  // that they are correctly saved as negatives if `ms` was originally
  // negative.
  var one = (ms < 0) ? -1 : 1;
  ms *= one;
  
  // calculate how many total days, hours, minutes, seconds add up to the given
  // ms delta by subtracting the biggest amount of days, hours, minute, then
  // seconds possible from the reamining milliseconds.
  _.each(['day', 'hour', 'minute', 'second'], 
    function (unit) {
      var factor = msPer[unit];
      // the 's' is for pluralizing the attributes on the delta object
      delta[unit + 's'] = Math.floor(ms / factor) * one;
      ms = ms % factor;
    }
  );
  return delta;
};