Show occurrence number of a recurring event

[yvolk]

I found good example cases here:

https://answers.microsoft.com/en-us/msoffice/forum/all/is-there-any-way-to-show-the-occurrence-number-of/47618036-12e2-4286-9826-444ee68aa4c6?auth=1

https://webapps.stackexchange.com/questions/10850/increment-number-inside-a-recurring-event-in-google-calendar

If the event is used for a person's birthday, this may be used to easily count age (in years, or even in weeks...). It's unclear yet, if we can reliably count occurences, but this worth a try...

See related discussion #227 (comment)