In The Pirates of Penzance, the Pirate King and Ruth inform Frederick that he is bound to serve them, not until his twenty-first year, but until his twenty-first birthday.
“Through some singular coincidence—I shouldn't be surprised if it were owing to the agency of an ill-natured fairy—
You are the victim of this clumsy arrangement, having been born in leap-year, on the twenty-ninth of February,
And so, by a simple arithmetical process, you'll easily discover,
That though you've lived 21 years, yet, if we go by birthdays, you're only 5 and a little bit over!”