title: iCalendar: MONTHLY RRULE without BYDAY or BYMONTHDAY
body: <p>How to interpret a monthly recurrence rule with only DTSTART (no BYDAY or BYMONTHDAY)?</p>
  
  <p>According to RFC5545 you can define a recurrence rule with a frequency only, for example  </p>
  
  <pre><code>DTSTART;TZID=US-Eastern:20180831T090000
  RRULE:FREQ=MONTHLY
  </code></pre>
  
  <p>A smartphone with LineageOG (Android Calendar) apparently interprets this as BYMONTHDAY and shows the event only if there is a 31st of a month. 
  An iPhone apparently interprets this as the last day of the month and shows the event each month (31st, 30th or even 28th). 
  Which interpretation is correct? </p>