Ab nächsten Monat bekomme ich ...
The prepostion ab should be followed by the Dative case, and the Starke Flexion applies here, so I'm tempted to say "ab nächstem Monat" instead of "ab nächsten Monat".
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Sign up to join this communityAb nächsten Monat bekomme ich ...
The prepostion ab should be followed by the Dative case, and the Starke Flexion applies here, so I'm tempted to say "ab nächstem Monat" instead of "ab nächsten Monat".
Two factors coincide:
The German dative is slowly dying, being replaced by other cases across the board. We may not see the last of it, but our descendants surely will.
No matter how you intend to pronounce it, in practice, "einen"+"Monat" is indistinguishable from "einem"+"Monat" anyway because of coarticulation. Therefore, people who spell "by ear" have even less incentive than usual to preserve the dative in this construction.
Actually, you may use both dative and accusative with ab in a temporary usage.
ab nächstem Monat
ab nächsten Monat
Duden uses the example
ab kommendem/(auch:) kommenden Montag
Note the accusative usage is restricted to the above shortened form. As soon as you introduce an article, like in
ab dem nächsten Montag
ab den nächsten Montag
accusative no longer works.