In the first season, (Cory's sixth grade year,) Amy and Alan mention that they've been married for 18 years. In season 2, (Cory's seventh grade year), they celebrate their 17th anniversary. In season 3, (Cory's eight grade year) Cory and Eric throw them a party for their 20th anniversary.
When Cory is in the high school hallway, he talks with Mr. Turner in order to invite him to his parents' anniversary party at Chubby's. He tells Mr. Turner that the party is in six hours. Later, he runs into Mr. Feeny and invites him to the party, telling him that it is at 8 o'clock that night. This would mean that it is currently 2 o'clock when Cory is talking to them. However, only moments later, Mr. Feeny runs into Reginald Fairfield and, trying to avoid having lunch with him, tells Reginald that he has a full morning of classes before lunch. This doesn't make sense as it is already 2 o'clock in the afternoon.
After having to pay to $10 to get into the party Mr. Turner tells Cory that that there is a $10 cover for tomorrow's English class. However, the party is on Friday night, so there would be no school the next day.