In Uncountable nouns^x, Noun phrases^x

The only thing that can come after a preposition to form a prepositional phrase is a noun phrase, usually a noun accompanied by its preceding determiner and perhaps an adjective. A relative clause may be added. The noun itself may be a gerund. If the element after the candidate for a...

For instance, "I don't have time to do this." Here, the noun "time" is uncountable. "I had a great time." In this case, is the noun in the noun phrase "a great time" countable? IS there any systematic way to explain the concept behind this? According to several grammar books I looked into,...