Tables :: Empty Date Field - Using As Unique Identifier In Index?

To prevent duplicate records, I use multiple fields indexes, which worked fine until now. I learned that each "empty" fields are consider unique by Access, so not the best in an index to prevent duplicate records. I managed to have it worked using the default value property to give each "empty" records the same value.

Now my problem is that I have a date field which is optional, but I need to use it as a unique identifier in an index. I could again use a default value, but since it has to be a date, I find it confusing. I would rather have a default value of "No date specified" or "-" or even better, "". Unfortunately none of those works with date fields.