Furthermore, [MS-OXOCAL] Section 2.2.1.41.1 indicates:
The wDay field is set to indicate
the occurrence of the day of the week within the month
(1 to 5, where 5 indicates the final occurrence during
the month if that day of the week does not occur 5 times).
[rule firstOccurrence] may return negative values according to iCal spec for
recurrent rules iCal 4.8.5.4 Recurrence Rule. For instance, for defining
a timezone whose recurrent rule is done using this rule:
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
This fixes the problem when editing a recurrent appointment in
Outlook were incorrectly shifted when SOGo provides back the event.
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