diffDates.js

function diffDates(startDate, endDate) {
  var t, multiplier = 1;
  if (startDate > endDate) {
    t = startDate;
    startDate = endDate;
    endDate = t;
    multiplier = -1;
  }

  var millisTotal = endDate - startDate,
      millis = millisTotal % 1000,
      secsTotal = Math.floor(millisTotal / 1000),
      secs = secsTotal % 60,
      minsTotal = Math.floor(secsTotal / 60),
      mins = minsTotal % 60,
      hoursTotal = Math.floor(minsTotal / 60),
      hours = hoursTotal % 24,
      daysTotal = Math.floor(hoursTotal / 24),
      weekDays = daysTotal % 7,
      weeksTotal = Math.floor(daysTotal / 7);

  var startDay = startDate.getDate(), endDay = endDate.getDate(),
      startMonth = startDate.getMonth(), endMonth = endDate.getMonth(),
      startYear = startDate.getFullYear(), endYear = endDate.getFullYear();

  var yearsTotal = endYear - startYear;
  t = new Date(+startDate);
  t.setFullYear(startYear + yearsTotal);
  if (t > endDate) {
    yearsTotal--;
  }

  var monthsTotal = yearsTotal * 12 + (startMonth > endMonth ? (12 - startMonth + endMonth) : (endMonth - startMonth));
  t = new Date(+startDate);
  t.setMonth(t.getMonth() + monthsTotal);
  if (t > endDate) {
    monthsTotal--;
  }
  else if(startMonth == endMonth && t.getFullYear() < endYear) {
    monthsTotal += 11;
  }
  var months = monthsTotal % 12;

  t = new Date(endYear, endMonth - (endDay < startDay ? 1 : 0), startDay, startDate.getHours(), startDate.getMinutes(), startDate.getSeconds(), startDate.getMilliseconds());
  if (t.getDate() != startDay) {
    t = new Date(t - t.getDate() * 24 * 60 * 60 * 1000);
  }
  var monthDays = Math.floor((endDate - t) / (24 * 60 * 60 * 1000));

  return {
    millisTotal: multiplier * millisTotal,
    millis: multiplier * millis,
    secsTotal: multiplier * secsTotal,
    secs: multiplier * secs,
    minsTotal: multiplier * minsTotal,
    mins: multiplier * mins,
    hoursTotal: multiplier * hoursTotal,
    hours: multiplier * hours,
    daysTotal: multiplier * daysTotal,
    weekDays: multiplier * weekDays,
    yearsTotal: multiplier * yearsTotal,
    monthsTotal: multiplier * monthsTotal,
    monthDays: multiplier * monthDays,
    months: multiplier * months,
    weeksTotal: multiplier * weeksTotal
  };
}