helper function for _.addTime and _.subtractTime
function addOrSubtractDurationFromMoment(mom, duration, isAdding, ignoreUpdateOffset) {
        var milliseconds = duration._milliseconds,
            days = duration._days,
            months = duration._months,
            minutes,
            hours,
            currentDate;

        if (milliseconds) {
            mom._d.setTime(+mom._d + milliseconds * isAdding);
        }
        // store the minutes and hours so we can restore them
        if (days || months) {
            minutes = mom.minute();
            hours = mom.hour();
        }
        if (days) {
            mom.date(mom.date() + days * isAdding);
        }
        if (months) {
            mom.month(mom.month() + months * isAdding);
        }
        if (milliseconds && !ignoreUpdateOffset) {
            moment.updateOffset(mom);
        }
        // restore the minutes and hours after possibly changing dst
        if (days || months) {
            mom.minute(minutes);
            mom.hour(hours);
        }
    }