Hello Davida,
We're sorry for the delayed response and inconvenience caused. As per the description above, when converting an excel file to PDF the date format in the PDF file is changing, is that correct?
Try exporting the PDF file through the URL https://cloud.acrobat.com/convertpdf with your Adobe ID and password on a browser like Chrome, or Firefox, or Safari and see if this brings any difference.
Ideally, the date format should not change, try exporting any other excel file to PDF and check. If it is specific to one excel file, is it possible to share the excel file and the converted PDF with changed date format with us so that we can test it at our end?
To share the file, please use Adobe Send feature, login to https://cloud.acrobat.com/send using your Adobe ID(email) and password, upload the file, share the link to files via private message only, How Do I Send Private Message
The record shows that you are subscribed to Adobe PDFPack services, install the trial of Acrobat Pro DC and create the PDF file from excel and see if this issue is reproducible Download Adobe Acrobat DC (Continuous) for Enterprise or VIP licenses
You can also try the suggestions from Adobe discussion Converting an Excel document to pdf format.
Let us know how it goes and share your observation.
Thanks,
Anand Sri.
[Edited response]