The problem is that each nested list also contains 6 items so the c matches the third sublist.
l6 = Range[36] // Partition[#, 6] &
l6 /. {a_, b_, c_, d_, e_, f_} :> c
(* {13, 14, 15, 16, 17, 18} *)
l5 = Most@l6
l5 /. {a_, b_, c_, d_, e_, f_} :> c
(* {3, 9, 15, 21, 27} *)
To avoid issues like this use more specific patterns
test /. {a_, b_, c_DateObject, d_, e_, f_} :> {a, b, DateObject[c, "Month"], d, e, f}
