Why did 🧙Gandalf mention Rivendell in advance?

[BlackCaptain]

I understand mosly everything but one part.:

Gandalf meets Aragorn around the beginging of May. He meets Radagast at the end of June. His letter to Frodo makes sense to me now. But when he is talking to Aragorn (Stirder tells the hobbits about this in his chapter), he sais "Frodo (or the Hobbits) should set out for rivindell at the end of September". or something like that. How does he already know that theyr'e gonna go to rivindedell? He doesnt even know about the ring yet, or that the Nazgul have set out from Morgul yet... does he?

 

[Aulë]

Yes- remember when he threw the ring in the fire? He had already been to Mnas Tirith, so he knew if the words appeared, it was the Great Ring.
He then told Frodo to leave for Rivendell, and was planning to meet him in Bree after he visited Saruman.

 

[Brent]

and he also says something about going no later than September and Frodo says he will go on Bilbos birthday (and "I shall take Sam to see the Elves he will be delighted") and Gandalf says no later than that

 

[Eriol]

Gandalf was in the Shire in April (when the chapter "The Shadow of the Past" takes place). I think this is the missing piece in your chronology