If you want to make it more in sync you will have to simulate all explored chunks, or at least the ones that you were last at.
I am sure you can understand why that would be difficult for some people, considering the fact that as your computer is simulating the last chunks you were in it is also rendering new chunks, as you move through the world, and updating the chunks you are currently exploring. And when you go to another planet, like mars, you will have to simulate the moon -and- the earth (not to mention any space stations you have) if you want them all to be in sync; and this will go on and on as you visit more planets until the last planet that you decide to visit.
So while it is a good idea, I question its' ability to be used in a standard home computer (not the beast I have, 50G ram anyone?) unless it is a feature you can customize; for example, being able to make it so it only simulates the -last- planet you were on, instead of all the planets you have visited.
Another approach, far less complicated (you wheat wont grow while you are gone), could be instead of simulating the chunks, to have a timer for each planet and when you enter the world have GC execute /time add <Timer_<WORLD>> so that it adds exactly how long you have been gone; this method, however, can also be a resource hog if there are like--- 20+ planets.
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Just throwing stuff out there to make you guys think!
I like the idea :]