There is evidence that Danian underwent a-mutation in some conditions, notably [u] became [o] before final [a] in cogn < ᴹ✶kuʒnā (Ety/KUƷ). There was a similar change in late Proto-Germanic (ref/@@@). Unlike the Noldorin and Ilkorin a-mutations, there is no evidence of the mutation of [i] to [e] before [a]. Unlike [u] > [o], such a-mutations were rare in the Germanic languages that inspired Danian, so Tolkien may have decided that the Danian a-mutation only applied to short [ŭ].

Since the resulting [ŏ] did not change further, this means the Danian a-mutation must have occurred after the change of short primitive [ŏ] became [a]. This also means that, where short [o] appeared in Danian, it was the result of the a-mutation of [ŭ], which was true of Proto-Germanic as well.

There is one example, Dan. dunna, where a-mutation is expected but does not occur. Perhaps like Ilkorin, the a-mutation was prevented or reverted before certain nasal combinations. The same was true of a-mutation in Proto-Germanic as well (ref/@@@).