Excel and Julian Date - has something change recently

Just my take on this, nothing has changed in Excel that I've heard of in this regard. Your image looks like a formatting issue that might be caused by a change in the source management software program, or maybe in the destination workbook that you're not aware of if it's a template of any kind. Until you find out why that is (if my suspicion is correct; maybe it's not), you can copy any empty cell, select the numeric range, paste special for Add in the Operation section, hit OK and then format for date as desired. I did that on a replica of your image data here. It's albeit a clunky workaround but something is going on in the export process that as far as I know, in my opinion, and speaking to your post's Subject line, is not due to a change in Excel.