I recently updated from CiviCRM 5.5(.2?) to 5.14.1, and now if I go to edit the membership record for a contact (or add a new membership record), the Member Since, Membership Start Date, and Membership End Date fields all show 12/31/1969. I can put correct dates in there and save the record just fine, but for an existing membership that I'm editing, it should (and used to) show the dates that are currently stored in the database, and which show up correctly on the contact's Membership tab.
I'm running it under Joomla 3.8.12 with PHP 7.1.1 on IIS 7.5. Honestly, I have no idea where to even start looking to figure out what the heck is going on.
Has anyone else seen this, or have any suggestions on where to start?