Even if they show you their birth certificate, identity card, passport, work card, green card, it’s possible you will see different dates on some of the official documents. And, the dates on those documents could likely be inaccurate. There was a time, before computers, when for a fee, in China you could have your birthday changed on the official records. Not sure why anyone would, but a few tried to explain that it allowed her father to retire earlier. But, I never understood why she would want to change her birthday. This coupled with the fact, that some are here under assumed names, or a relatives name who certainly doesn’t have the same birthday as they do ensure that you probably aren’t going to know their real birthday. Case in point, a friend married a provider for love. No money, no green card, just plain simple love. He came to find out years later the name she used and told him was not her real name; naturally the birthday was false too. He just laughed it off and said, what does it matter?