Because it’s Leap Day, February 29!

In one of the funniest songs in The Pirates of Penzance, the Pirate King explains that young Frederick, having been born on Leap Day, may have been alive for 21 years — yet, if one goes by birthdays, he’s only 5!

 
After they sing, they have the following discussion:
 
Frederic. Upon my word, this is most curious – most absurdly whimsical. Five-and-a-quarter! No one would think it to look at me!

Ruth. You are glad now, I’ll be bound, that you spared us. You would never have forgiven yourself when you discovered that you had killed two of your comrades.

Frederic. My comrades?

King. (rises) I’m afraid you don’t appreciate the delicacy of your position: You were apprenticed to us –

Frederic. Until I reached my twenty-first year.

King. No, until you reached your twenty-first birthday (producing document), and, going by birthdays, you are as yet only five-and-a-quarter.

Frederic. You don’t mean to say you are going to hold me to that?

King. No, we merely remind you of the fact, and leave the rest to your sense of duty.

And there you have it — the reason why Pirates’ subtitle is “The Slave of Duty.”