I frequently use calendar to log when I have done something or when something happened. In these cases, it would be helpful to have an option to add a notification after an event, in the same way that there is currently an option for notifications before an event.

For example, if I had a cleaning task which needed completing once per month, I could create an event on the day I completed this cleaning task, then set a notification for one month after this event, to remind myself that this task is due to be repeated.