IIRC there's a rule about not being able to change any of the Secure Flight information within 72 hours of departure. Hence you cannot change the birthdate yourself. I don't know whether phone agents can override this or whether it requires document verification at the airport. It's a rule that applies if you're trying to add a Know Traveler Number, but I've never heard of a birthdate being wrong somehow.

It's a good practice to check this online about five days before any international itinerary, especially if you're dealing with a flakey computer system or one that's had some major "updates" or "enhancements" recently.