Physcists say that the universe is approx. 13 billion years old. However, the amount of time that has elapsed since the big bang differs per object, dwpending on the trajectory that that object has travelled through spacetime (so far as I understand).
So what does the number 13 billion refer to?
Is it the time that has elapsed for a reference frame that has followed the earth (i.e. the particles that make up the earth)?