I don't believe this will work. changing your birthday on your Samsung account is highly unlikely to allow you to receive a birthday code sooner, as Samsung typically sends these codes based on your registered birthday and may have systems in place to prevent manipulation of this information; it's best to keep your birthday accurate and wait for the code to be sent around the correct time
Also there is no guarantee of a Birthday discount code. I am missing discussed on here many time, but I many have never received a code, sent some every year on the dot.