He receives a letter telling him to go to a place that he would never think of going to--to just drive to a place he didn't know existed. When he does he sees that that part of the world (part of Texas?) hasn't been created yet--it's just a blueprint or something--just green lines of a simulation unfinished? He then goes on a hunt to find out the truth about the created world he lives in. He eventually meets someone in a tower in another world or simulation that has been controlling his simulation only to find out that everything is simulated?
This sounds a litle like the movie "The Truman Show" starring Jim Carry. He was living in a completely simulated world, weather and all.
There a number of movies explorig this theme.
In addition to the given answers, exitenz by david croneburg is worth seeing. A lot of his films touch upon this questioning of reality.
Jacobs ladder, although not sci-fi is also good.
it's the 13th Floor. I just watched it. The time travel principal you remember is the fact that different world simulations are simulations of a different time. 1990s, 1937 and 2024