Of course you also need to know the month, but similar to the year i would argue that there are plenty of times where the month is evident from context. So the informational value is lower than the day.
I don’t want to argue that this is an absolute thing, but i’d say that quantitatively there are more times where you only need the day compared to very few times where you only need the month for example.
For me the bigger value is not in the quality difference between the two platforms. And don’t get me wrong, i agree that BlueSky is a lot better than Elon’s Twitter, but not as good as a decentralised Fediverse Platform.
The real positive is in the act of migration itself, because it shows that is still a possibility. So hopefully it proves sustainable.