Changing recurrence rule messes up past events

[rnixx]

When changing the recurrence rule of an event where events defined by the old recurrence rule already happened, data gets inconsistent - the dates of the past events change respective are not re-constructable any longer (event occurrences are volatile objects)

a way to work around this could be a check each time an event object gets modified whether the start/end/recurrence rule changed and already happened occurrences start and end dates get stored on annotations (or on some field which not gets displayed in edit form)

this data could then be considered when calculating occurrences and we would have consistent and reliable event data.

such a change would probably also imply changes in the underlying date indexing mechanism.