If it’s out of a rescue group, its going to be made up anyway. They almost never know the real dog age, just a good guess. They would have to be an abandoned litter, and even that probably has an error margin of a few days to a week depending on how young they are.
I know how old my cat is because I know how old my niece is. If you’re bad with dates make them easy to remember. Not going to lie i make anniversaries intentionally on the first and not an insignificant amount have been Jan first which is even easier.