Could someone explain me this joke, please? Thank you!

Three elderly men are at the doctors office for a memory test.

The doctor asks the first man, “What is 3×3?”
“274”, came the reply.

The doctor rolls his eyes and looks up at the ceiling and says to the second man, “It’s your turn. What’s 3×3?”
“Tuesday”, replies the second man.

The doctor shakes his head sadly then asks the third man, “Okay, your turn. What’s 3×3?”
“Nine”, says the third man.
“That’s great!” says the doctor. “How did you get that?”
“Simple”, he says. “Just subtract 274 from Tuesday.”