"Fland Canyon" and Maude

Based on your comment:

@Walt I agree, but such a significant event surely must lie on a static timeline.

There is no static timeline, at least not to the degree that you're expecting it to exist. Acknowleding that Maude died 15 years ago would require acknowledging that Bart (10), Lisa (8) and Maggie (1) never knew Maude. More worryingly, Rod (10) and Todd (8) Flanders could not be Maude's children then either.

Clearly, you cannot define the in-universe timeline by the out-of-universe timeline. Just because the episode in which Maude dies aired 19 years ago (16 years when you posted the question), does not mean that 19 (or 16) years have passed in the Simpson universe.

If you define the timeline based on the characters' ages, it becomes clear that no one has aged in all this time. Bart is still 10, Lisa is still 8.

There is no real timeline for the show. It's intentionally kept very loose to avoid the writers having to stick by a static timeline that would hamper jokes and become a general nuisance which doesn't add anything of value to the show.

I think there's a simpler explanation than "Simpsons doesn't have a static timeline" (which is true).

The show's episodes that ran from 2000 to 2016 simply depicted 2 years worth of the life of the Simpsons. Other than episodes which show special events such as birthdays and holidays, and episodes that span a longer period of time, it's reasonable to think that each episode takes place a day or so after the previous episode.

So there is nothing wrong with thinking that the grand canyon episode took place less than 2 years after the Maude's death episode. Just because it took 17 years in between airing each episode doesn't mean that 17 years have passed.