code: function startOfMonth() {
        var result = this.clone();
        result.day = 1;
        result.isDate = true;
        result.hour = 0;
        result.minute = 0;
        result.second = 0;
        return result;
      }
docs: Returns a copy of the current date/time, rewound to the start of the
  month. The resulting ICAL.Time instance is of icaltype date, even if
  this is a date-time.
  
  @return {ICAL.Time}      The start of the month (cloned)