according to the wiki, theres a different president in each of the three original trilogy of films (unnamed in x1, mckenna in x2, and another different unnamed for x3); so i suppose the the president from x1 was finishing his first (or second) term, which was served from 2000-2004, then mckenna wins that november, meaning x2 must be late january or at least within the early months of 2005 (to keep it within a short time frame of x1), then he's impeached or otherwise somehow replaced by the time of x3 in 2006
…unless we want to assume that the original president of the us as seen in x1 is replaced by his vp (mckenna) very shortly before the 2004 election, between the events of x1 in late sept 2004 and that november, placing the events of x2 in late 2004 and still allowing the president of x3 to be elected but not yet having taken office in january 2005
…or the wiki is wrong and mckenna was president for the first two movies