as a pre-noun adjectival is used to mean "one" or "a certain," in common constructions such as .
In this case, is clearly bringing some "baggage" with it in the form of:
>
(Where the is converted from ). So you can be certain that is acting as the verb in a phrase modifying the noun .
Having said that, I don't think there's any strict difference between these two uses of . While may sound like, e.g., , it's simply "a day that exists," using the verb