Last_purchase_date Problem

In my exported csv, I noticed that there is some variation in the way that last_purchase_date is formatted. E.g:
29-09-2017
29-09-2017
2/10/2017
2/10/2017

This appears to change over time and is not related to me updating the records.
For example, people who registered this month have m/d/yyyy, whereas the previous 3 weeks are formatted dd-mm-yyyy, which Excel won’t recognize as a date. Looking at the whole membership list, I see this began in mid-March and has changed periodically since that time. The other date fields do not have this formatting problem.
What is the correct format? Any insights or fixes?
Thanks.

Any changes in the site and server meantime? Settings of date format on your site and in the DB engine is first that coming to my mind…

The formatting of these values changes frequently. I fix them for the memberships which I upgrade manually. We are not changing the date format on our site.
Thanks.