Q: searching for characters by birthday

I'm working on a project for a web dev course, and we are required to use two APIs, and we settled on this one as one of them. We're having trouble successfully searching for a character by birth date though, which the documentation suggests should be possible. I've tried looking up a character (e.g. Aquaman) to make sure there is a birthday on file for them before trying to do a /character call or a /characters call, and since the documentation doesn't mention how the date is formatted I've tried all sorts of formats; in particular one update to the API suggests YYYY-MM-DD, but that doesn't seem to work. Is there something obvious I am missing? Is searching by birthday impractical/should we go for something else?