Muttley's Birthday is on the 5th of April. (As seen on the calendar is holding twice in this episode)
Dick Dastardly: We've been batting zero with your dumb inventions, Klunk. So, now, we'll try one of my dumb inventions.
Dick Dastardly: Well, Zilly?
Zilly(shivering with fear ): I'll stay here and cook a birthday cake.
Dick Dastardly: You come with me or i'll cook your goose, you chicken!
We learn that Muttley's birthday is April 16th. (MOD. NOTE: As is my younger brother's!)