How to change end date in Set Property After Alert

I’m trying to make a notification for the recurring task and here is the use case.

  1. Notification is triggered 1 day before the exp_end_date.
  2. Task assignees get an email notification
  3. Based on Set Property After Alert for exp_end_date the value is increased for 7 days

So, each week, 1 day before the deadline task assignees receive an email notification and task exp_end_date is moved for next week.

My question is what would be the proper value (formula) to make this calculation that will change the exp_end_date.

I’ve been testing this notification with task status change, but whatever I’ve tried, when I change the status of a task I get the error message that there was already a change in the task and I can’t save the status change. What happens is that system changes the date to e.g. 2010 between the point where I change the task status and hitting the Save button.