Hi StephanB! The Public API should return all dates in ISO-8601 format: https://api.qualtrics.com/instructions/docs/Instructions/dates-and-times.md; however some APIs were not following this standard and were returning in the incorrect format. Our developers have since fixed this to the correct format. The team is working on a process to improve communication of breaking changes.

Hi LaurenK, thank you for your reply which is highly appreciated. The link is a helpful one as well.
Unfortunately, your developers had changed it again. I was not amused on 11 Dec 2020: Since then, the API response of the two requests changed back. All error troubleshooting we did on 1 Dec 2020 after the initial change was not enough as we were running into an error again .
The documentation at https://api.qualtrics.com/api-reference/reference/mailingLists.json/paths/~1directories~1%7BdirectoryId%7D~1mailinglists/get is again wrong, as it shows string date/time for this request but it returns Unix milliseonds as integer.
The documentation at https://api.qualtrics.com/api-reference/reference/mailingListContacts.json/paths/~1directories~1%7BdirectoryId%7D~1mailinglists~1%7BmailingListId%7D~1contacts~1%7BcontactId%7D/get is correct again as the request does return int64.
So, an improval of communication would be important. How should we automate any API processes if Qualtrics changes the API without any proactive information?! This is not acceptable imho. Thank you!

https://www.qualtrics.com/community/discussion/comment/33708#Comment_33708StephanB - I've relayed this feedback over to the team!