Number of days instead of fixed date for registration and unsubscription

I would like to enter a number of days instead of a fixed date for event registration and unsubscription. When adding events we normally accept bookings until the event start date and we normally accept unsubscription until 7 days before the event.

It is time-consuming to manually select dates for each event. Also when we copy an event, it is easy to forget to change the dates. One solution could be to enter "7 days before start date" instead of a specific date and time.