When the day comes that humanity does have an intergalactic empire, there is one everyday item we will have to get rid of: calendars. Thanks to the Theory of Relativity, we know time is flexible. It moves at a different rate depending on your velocity.
Space ship navigators traveling at speeds approaching or somehow exceeding the speed of light will be very aware of time dilation. In the year it takes to go from point A to point B, several decades may have passed for the rest of the universe.
In order for science fiction writers to avoid the complex calculations in Einstein’s theory, there are several websites (listed below) that offer Relativity Calculators. Time doesn’t severely distort until around 80 or 90% of the speed of light, but even at 50% the crew of a spaceship will start losing a few hours per day.
At the moment, I’m not writing anything about space travel of this sort. My current project is much stranger. But if I were, I’d mention something about how time dilation has affected the lives of my characters. After a year at sea, the sailors of old could never really go home; how much worse would it be for a time dilated navigator.
My advice to fellow sci-fi writers is not to think too much about this problem. It is important, and space faring societies will be aware of it, but I’ve found that the more I try to explain Relativity the less sense it makes. So for your readers’ sake and for your own sanity, acknowledge time dilation exists, use a Relativity Calculator, and move on with the story. Leave the details for textbooks.
Relativity Time Dilation Calculator (The easiest to use, in my opinion).
Relativity Calculator (Allows you to choose different units of measure).
Space Math (Also has calculators for escape velocity, orbital periods, etc…).
Clark, Ronald W. Einstein: The Life and Times. NY: Harper Collins, 1971.
Krauss, Lawrence M. The Physics of Star Trek. NY: Harper Collins, 1995.