For the person who said it doesn't really matter when the game takes place, I suggest that it does. Roleplay games are about immersion. They present you with an alternative world, in which you have an alternative persona. The degree to which the artificial world "makes sense" will strongly affect your ability to roleplay within it. Every time you trip over a logical or stylistic inconsistency, you are wrenched out of character and forced to think "Well, I guess they messed that up." Determining, and determinedly sticking to, a time line for the game can help lead to immersion via better world building.
For a completely random example, the food. There is perfectly edible food available almost anywhere. I mean who doesn't enjoy the refreshing crunch and mouth-watering taste of Sugar Bombs? But if this is 200 years later, why is all this food in perfect edible condition? It's never explained in the game. You can try to explain it away (it's vacuum-packed food that was then irradiated to kill off microculture that was packed with the food) but having to step out of the game and apply real life to it so that it makes sense is damaging to immersion. You're on your own as far as explaining why nobody else has found and devoured this eternal cornucopia, or why the boxes themselves haven't rotted away from the sealed interior packaging. Why there is Grilled Mantis everywhere in the game world, despite mantids only showing up in some rare areas? And the fact that you never see anybody living in the area where you find this fresh food so why is it there, and the fact that it's not spoiled despite being unfrigerated, and the fact that you never see anybody cooking mantis legs, and the fact that you never see anybody hunting mantids. Where are these people getting all this mantis meat, why doesn't it rot, why do they prepare it in secret, and is this even mantis at all?!?
When you get to thinking about, F:NV is practically a psychological horror game in some ways.
Obviously not all of that has to do with time alone, but it's an example of how a world designer needs to carefully consider many aspects of what they are doing lest they create a visually appealing setting that really makes no sense at all. Time is a huge but subtle part of everything.