title: RRULE 20 days after every 15th in a month,in icalendar (RFC 5545）？
body: <p>I'm searching a way to express a situation like this in icalendar: an event happens every month, x days after a given date.</p>
  
  <p>e.g.: 20 days after 15th day in every month. so, it might be 3rd, 4th, 5th,6th( like February )
  if bymonthday can be set to 35, it's ok.</p>
  
  <p>But in outlook this won't work.
  How to solve this question?</p>