Alan Watts made same argument about when a person is born. Was it at partuition? Was it when the baby was conceived? Was it even further back, was it that moment in time when your father first saw your mother? The answer is what we declare as someone's birthday is completely arbitrary and for legal reasons we agree on partuition but nature doesn't care because to nature everything is all ONE EVENT. From big bang to right now, dividing it up into separate events is just a human thing.