Update:
According to this post: What timezone are dates in API data? - #2 by FredericMalenfant due dates are usually sent in the timezone of the card creator.
If so, I’m not sure how to figure out the certain user’s timezone, I’ve checked out what a member object is: https://developer.atlassian.com/cloud/trello/rest/api-group-members/#api-members-id-get
But I’m can’t find a time related property.