I don't want to insert something in the main chronology without first checking in with the idea. I think another clue as to the year(s) in which the novel's events take place is in using the millennial cut-off as a point of demarcation for subsidized time. The Year of the Whopper would coincide with the new millennium, which, depending on whether you recognize AD years as beginning in Year 0 or Year 1, would have the new millennium beginning on Jan. 1, 2000 or Jan. 1 2001. I think it more likely that YW was 2000. DFW would certainly be aware that the millennium's actual start would be Jan. 1, 2001. But he would also recognize that our society places more weight on the symbolic importance of going from 1999 to 2000, than the chronological correctness of counting the years "since Christ" as the beginning of Year 1. So if YW is 2000, then YG is 2008, which corresponds with one of the possible ages of Matty Pemulis.
Note that DFW makes use of the term "post-millennial" numerous times, albeit infrequently, given the length of the novel. In interviews from the 90's, DFW was cognizant of a post-modern era and felt, I felt, a special significance approaching with the new millennium (Google "DFW interview millennium" for examples). I think there's a very strong inference that the inauguration of subsidized time coincides with the new millennium and I would suggest adding a sentence or two to the main chronology in support of the Matty Pemulis-based date(s) putting YG in 2008 or 2009. --Mimesis