Comment by carbocation

Using Google Books, you can quickly find works much older than the 1922 citation in this case, and which contain "Happy Birthday to you" in its modern form. Here, for example, is one from 1911:

> https://books.google.com/books?id=TfoaAAAAYAAJ&pg=PA63&dq=%2...

    "Happy birthday to you,
     Happy birthday to you,
     Happy birthday, Dear John,
     Happy birthday to you."

It even says that it's sung to the "Good Morning" tune.