↑"Brother, Can You Spare a Clarinet?" - Arthur says his birthday (May 25th) isn't for 7 more months, meaning the episode takes place in October. When they give Binky the repaired clarinet, Binky says his birthday is next month.
↑In "Buster Isn't Buying It", Arthur says that Buster's horoscope sign is Aquarius, which is typically between January 21 and February 18
↑"Fernlets by Fern" - As Mother's Day is the 2nd Sunday in May, it gives enough space for her birthday on the 16th to be in the same month. This makes sense, because it shortens the span of the episode from 1 month to 1 week.
↑"Dad's Dessert Dilemma" - Arthur mentions that her birthday is coming up soon, after Galileo's birthday, which is February 15. Also, this was two days after February 15 so it is not long after February 17.