Is there a function which can convert the RecurrenceRule and Start Time to Index value?

[cywan1998]

input: Recurrence rule, Recurrence Start Time, Occurrence RecurrenceId(Occurrence Start Time).
output: the index of the Occurrence in the recurrence series

[benfortuna]

Could you try using Recur.getDates with inputs: recurStartTime, occurrenceStartTime, etc.

You could then use indexOf on the returned DateList to find the index of the occurence.