Applies to: Starter, Pro, Plus, & Enterprise Plans
As you already know, when you or one of your Admins authors a Course, the choice can be made between: a fixed Expiry Date that governs the expiration of any and all Learner Enrollments; or, a fixed number of available days that starts to count down, as soon as an individual Learner is Enrolled in your Course.
But once you get to the Edit Activity workspace however, this second option is no longer available. If you want to set a different expiration limit - it must be a calendar date, only.
This makes a lot of sense, because if you have made the effort to open a Learner's Course Activity - it is very likely that you know exactly how much time you want to give them before their Enrollment expires.
Besides, the extra work of looking up when your Learner first Enrolled in the Course and then figuring out the right number of days to make their Enrollment expire on the date you want, is not a good use of your time - so it is simply not made available here at all.