DateString doesn't evaluate within Dataset

Question

[11.1]

Starting with a DateObject in a Dataset:

data = Dataset[<|"a" -> Now|>]

and then applying DateString

d = data /. date_DateObject :> DateString[date]

we find that d["a"] evaluates to a String. However, Normal[d]gives <|"a" -> DateString[DateObject[...]]|>. So I have two questions:

1. Why doesn't DateString[DateObject[...]] evaluate to a String within the Dataset or the Association?
2. Is there a way to force it to evaluate upon application of the ReplaceAll, so that the value of "a" in the Dataset and the Association is a String?