[Boromir88]

Let's look at the full sentence, maybe that can get us somewhere farther here.

Slow, because they do not count the running years, not for themselves.

Ok, the first phrase "they do not count the running years," then it follows up with, "not for themselves." So, now where I'm going is, the added "not for themselves," means they DO count the running years, they just don't do it for themselves. It's clear that some elves do count the running years. This sentence just says, they don't count the running years for themselves. Time is not important to the elves, but it's important if they wish to communicate with the other races, so there for they DON'T count it for themselves, but they DO count it in order to gather news from other places. Maybe the phrase is suggesting, they DON'T count the years for themselves, since they are ageless, and it's suggesting that they just don't count "how old they are," but they keep track of time to communicate with other peoples.

Therefor, I come to the conclusion that "do not" and "need not" are interchangeable, if you take it the way I just showed. If the sentence was just "They do not coun the running years," then that would mean they don't count it. But, it adds in the "not for themselves," which shows they don't count it for themselves, but they do count it for other reasons.

Edit: I know I just contradicted myself, but that's because I wasn't looking at the whole sentence before. I think the added "not for themselves," changes the meaning of the sentence. Making "do not," and "need not," interchangeable, since they don't count it for themselves.