The Gregorian calendar, the one we use now, is pretty accurate. But, before it's existence, people celebrated birthdays at the wrong time. The earth was not in the same place it was when they were born. But now, with our fairly accurate calendar, it is pretty close to the same place as when they were born on their birthday. Or is it?

No. it is not. The entire solar system is moving. The entire galaxy is moving at astronomical speeds as is the entire cosmos. We will never be in the same place. Ever.

A true birthday would require me to return to the same place I was born.

I keep moving forward.