What if I don't want to correct my girlfriend on my birth date?
Your gf doesn't know your real birthdate?
No, I want my girlfriend to know that they're making a mistake and she doesn't trust me
You should just tell her your correct birth date. Why all the secrecy? Are you hiding your real age?
Why did I want to hide it from my girlfriend?
You should not lie to your girlfriend about your correct date of birth. Eventually she will find out and this may change her view of who you are as a person. It would be right to correct the misinformation.