When creating a recurring series according to the rrule, this integer specifies whether to include the dtstart date if the date does not follow the rrules. For example, if on a Monday, you were creating a recurring series of meetings that were to be held every Wednesday, the dtstart would be Monday, but that does not fit the set of dates (all Wednesdays) generated using the rrules. Therefore the server must decide whether to include the dtstart date or not based on the value of excludedtstart.
A value of 0 indicates the dtstart date is included in the recurring series and a value of 1 indicates the dtstart date is not included in the recurring series. The default is 0.