Were these the dates you saw when you imported your metadata into Excel or did you enter new dates?
When I open exported metadata in Excel, Date created is in this form:
This is the form it should be. XMP requires this. Bridge will display the date in a prettier form, but the actual data needs to be "YYYY:MM:DD HH:MM:SS" with time in 24-hour format.
If you change the Date Created in the spreadsheet, you have to use the YYYY:MM:DD HH:MM:SS form.
Do you still have your original export .txt file? If so, you get those original dates and put them in an import spreadsheet, making sure they are formatted correctly. How do you do that? When you open the .txt file in Excel, format the date column as text, not a date. Excel wants to reformat to it's own date. I usually do not see this happen, so it might that you are opening the Excel data differently.
It would also be possible to use a script or Exiftool to copy the Exif Date Time Original to the IPTC Date Created, saving you a lot of manual work.
Another option for future Excel imports: delete the entire Date Created column from the spreadsheet. This way no date will be imported back in, eliminating the date format problem.
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