Data Validation; Date OR N/A

Working on a table, can you do data validation for a date OR n/a? For example, I have certification expiration dates, some expire 3 years after distribution, some never expire. Thanks in advance!

Hey! Thanks for the response, apologies on the clarity.

I have an excel table that will be filled out via a form. It will be used to track employee information and certifications. Many of the columns are "Expiration Date" in regards to certifications employees may or may not have. I am hoping to limit answers to a specific date, or "n/a" (not applicable). This is because I'm using conditional formatting to warn us when expirations are coming due. I don't want to leave blank, for many reasons. In data validation, I see that I can provide an error message if an entry is not a date, but I don't see a way to provide a "n/a" option as well.