The story starts with Bart & Lisa returning from Kamp Krusty, which is a summer camp. As such, the latest they would be returning home is mid-August. However, later Homer declares it's 3 months before Mother's Day, putting them in February (closer to when this actually aired). Either Bart has been managing to talk his way into missing school for more than half the school year (and Homer has been a model employee for the same length of time), or this is a mistake. While it's possible that Homer is mixed up, this is while he's been at his most efficient, effective and thoughtful - said Mother's Day treat being part of that - making it unlikely he'd make such an obvious mistake at this time.
The story's description in guides refers to Kamp Krusty as "Kamp Krustier", most likely after the episode title, which is actually just a reference to the fact that this is a sequel to the episode "Kamp Krusty" from Season 4 (much like the sequel to Die Hard was called Die Harder, which most likely actually inspired the title choice)
As a sequel to Season 4 Episode 1 Kamp Krusty (1992), which was the season opener in 1992, this is ostensibly set in Fall 1992. However, there are several indicators this is a modern story, such as Homer's suggestion to subscribe to a magazine using a new email address, when signing up for something using an email address wasn't available back then.
This shows Nelson getting off the bus returning from camp Krusty. However when that first aired in 92 Nelson wasn't at the camp.