It's not "earth" time they would have been modified back to Darren, but solar time. Both solar and sidereal time measure the time on earth, but the mechanism they use to do it is a little different in each case. They can be applied on other planets too, it's just that earth will always be where it's at for the vast mojority of us of course.

The Wikipedia article is a good starting point if you want to understand a bit more about sidereal time:

http://en.wikipedia.org/wiki/Sidereal_time