[stephen_g]

Interesting. As a native English speaker, I didn't even consider "before Cheryl had told me, I did not know" as a possible interpretation, just from the way the conversation flows but also because it goes without saying and wouldn't add anything.

In the context of Albert's statement though, it couldn't mean that - Albert's statement is saying that Bernard does not have enough information to determine the date just based on what Cheryl told him, so in saying "at first I did not know, but now I know", the only difference between "at first" and "now" is Albert speaking.

So Bernard's comment could only mean, "Based on what Cheryl told me, I did not know what the birthday was, but given the extra information I have as a result of Albert's statement, now I know"