Here's a problem I haven't seen discussed anywhere before. The basic question is: "How should time be kept on multiple planets?" At first glance this might seem like a simple question with a simple answer - "Duh, just divide up the planet's day into 24 hours and go from there!" I disagree.

Let's take an extreme example: Humans have colonized a new world. Measured by our standards the planet rotates once every 36.5 hours and has a 451 day year. If you simply divide up the day into 24 hours you end up with a second that's a little over 1.5 standard seconds long - unacceptable. What's the solution?
One possibility is to continue to use our standard length of time and to simply declare the day to be 36 hours, 31 minutes, and 18.5 seconds long. At the strike of 3621:18.5 a new day begins. Confusing - yes, but it could work.

Now let's complicate things even farther. As an author, I find it boring to write a book where humans are the only life forms in the galaxy. So now what happens when multiple species, each with their own unique timekeeping system, decide to colonize a new planet together? How should time be kept? It's doubtful they would use hours, minutes, and seconds. I doubt they would use weeks and months either. Their method of keeping time would be just as alien as they are to us.

One more thing to consider - in any star-spanning civilization the need to compare events across space will occur. The news might report that Earth's President was newly elected on February 10th, 2155 (Earth time). The population of a human colony world might want to know what that translates into in their local timekeeping system. How can so many planets keep their historical records in synch? (Sorry Einstein, I'm going to ignore simultaneity issues for now.)

Here's one proposal but it's far from perfect: All the races agree to a central timekeeping standard. Each planet maintains two timekeeping systems; local and galactic.

As a side note - a savvy business traveler making stops on multiple worlds would have to have a sophisticated, programmable timekeeping device in order not to be late for his meetings!
I've started an SFF discussion concerning this at:  http://www.sffworld.com/forums/showthread.php?p=690410#post690410