Everything I Know About Time I Learned From Steve Miller

I was just reading A Brief History of Nearly Everything, particularly a chapter that dealt with the age of the universe -- where I'm at right now some scientist or other has speculated that the universe is 20 billion years old. And it got me to thinking -- probably too simplistically -- but it got me to thinking, how can we really think about "years" on a scale like that?

Because a "year" is the time it takes for the earth to travel around the sun. But if the earth is, what, 4.5 billion years old, or something like that -- how can we measure years before the earth existed? If there was no earth to travel around the sun, I mean. If "time" is relative to gravity, if time is relative to movement through space -- I don't know if we can think about "20 billion years" once we're talking about something off of the planet earth, can we? I mean relative to us, sure -- it takes 76 years for Haley's Comet to swing within viewing distance of the earth. But that's 76 years for *us* -- if we were to attach a pocket watch to Haley's Comet, would it calculate the same amount of time?

I think I've read that if you take an airplane from, say, New York to Los Angeles -- when you get off that plane you're a little younger than the folks you left behind in New York, by virtue of the speed you've traveled through space. Space meaning physical space, not outer space. So if even *that* movement alters one's own perception of time...

I guess ultimately what I'm saying is, how can we talk about "20 billion years" when there haven't BEEN 20 billion years -- meaning, 20 billion passages around the sun by the earth?

And somewhat separate -- what if the earth has made some passages around the sun, or WILL make some passages around the sun, either faster or slower? Is it still a "year"? Are we really talking about "20 billion theoretical revolutions around the sun"? We already have a concept of a "Martian day," since it's something like 23.5 hours as compared to our 24 hours ...

I realize I'm thinking myself in circles, and I just got out of bed to type this as a means to not forgetting all about it in the morning. I'm not a stupid guy, but once you start talking about physics or astronomy I get easily confused. I'll just post this, move it to the back of my brain for now, and then go back to reading.