That is correct. The law is similar in Georgia. The judge tells you during sentencing, that any commission of a new offense is a violation of your probation. That is a condition that is placed on you from that date of sentencing. If you are ultimately convicted for an act that occurred before you were sentenced to probation, it cannot be used as a basis to revoke your probation.
I don't practice law in your state, but generally speaking a conviction, during probation, for an offense that was committed before the defendant was placed on probation, is not considered a violation of probation. Though it might be different in GA, I don't imagine it is.