It seems that the only time these fields can be populated is if the enrollment object is created via API, not CSV or GUI. Here is the field w/ comments:
start_at (datetime, optional): The start time of the enrollment, in ISO8601 format.,
end_at (datetime, optional): The end time of the enrollment, in ISO8601 format.,
Enrollments - Canvas LMS REST API Documentation
The SIS csv does not list support for start/end date at the enrollment object level:
SIS Import Format Documentation - Canvas LMS REST API Documentation
For us, this works fine as our start/end dates are course and/or term based and not specific to each user. Canvas does provide these fields in CSV for courses and terms. All our enrollment start/end dates are null like your example.
There is very limited documentation on what the enrollment start/end date would do if populated, I would guess it would override the start/end date of the course for just that enrollment. You could test out by creating an enrollment via API and populating those fields.
If you are not needing any user specific start/end dates, I would suggest sticking with the course/term start/end dates.
Thanks,
Anthony Hess
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