Explaining a billion years to a 7 year old

While many people are trying to figure out how to explain the Sandy Hook shooting to their children, I'm struggling to explain something a little different.

A few nights ago I watched a documentary on Mysteries of the Universe with my 7 year old, not realized that the show ended with a discussion of how the universe might end. We are given 3 likely options: big freeze, big rip, or big crunch. Each of these are likely to occur tens of billions to trillions of years in the future.

Since then I have been asked at least twice when we needed to plan for the end of the universe, or if we needed to worry about it. As much as I'd like to say "Don't worry your little head about it." I don't want to be so condescending and shut off discussion.  So I explain that I am around 1 billion seconds old. I hope this helps her understand these are far far off in the future.  But I am very proud of how calmly she can wonder about the end of everything.