recurrence
Concatenates, if necessary, multiple RDATE/EXDATE lists and converts them to a formatted string which OpenTasks can process. OpenTasks expect a list of RFC 5545 DATE ("yyyymmdd") or DATE-TIME ("yyyymmddZ") values, where the time zone is stored in a separate field.
Return
formatted string for Android calendar provider
Parameters
dates
one more more lists of RDATE or EXDATE
tz
output time zone (null for all-day event)