It depends on the process that you use to upload them into your database.
If you save a Text file (like .txt, .prn, or .csv), try reformatting, doing the
SaveAs and open the text file in Notepad to verify.
If your importing procedure reads the excel file, I would think that it would be
better to make sure that it knows how to read dates--and handles them correctly
itself.
If the importing procedure reads the field as text (while in excel), you could
use a helper column with a formula like:
=text(a1,"yyyy/mm/dd")
and drag down
Copy|paste special|values and delete???? the original field.
But that won't work if the original data isn't a real date.