True, but Portal is more about space than time. If you think about all the great games that had different times/eras, each time was in it's own seperate area;
Chrono Trigger, Prince of Persia: Warrior Within, Timesplitters...
I'd say it would take a lot of effort to get it working perfectly, but I won't say it's impossible. Imagine an Oddworld game with time travel, you could go back and save the mudokons if you accidently kill them
(like quicksave!).