The "Grosse Woerterbuch der deutsche Sprache" by the "Duden-Redaktion" notes that "ab" is sometimes used with the accusative case if
(a) "ab" is used as a temporal preposition AND
(b) it is followed by (adjective + noun) without any article.
Here is an example from the Duden grammar, which is aware of this phenomenon, too:
"ab erstem Mai" (dative) / "ab ersten Mai" (accusative)
In contrast to the classical "Wechselpraepositionen", there is no difference in meaning between "ab" with dative and "ab" with the accusative case. So I think it's easier to always use the dative case, which is never wrong.
(There are some more prepositions that - under specific circumstances - can be used with two different cases, namely "dank", "laut", "trotz", and "wegen", which occur with both the genitive and the dative case without any change in meaning. - Note, however, that specific rules apply. "Wegen", for example, is used with the dative case only colloquially, mostly among not so well educated speakers. So again, I suggest you always use the standard textbook case with these prepositions, which will never be wrong.)