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Triennial
A normal year has 365 Days. Once every 4 years there is a leap year, which is 366 days. If you don't count leap years as special then 12 years = 4380 Days If you count leap years, then 12 years = 4383 Days (3 extra leap year days)
Simple. Being born on a leap year on the 29th of February. As the 29th of February only occurs once every four years, you would have to live for 400 years to actually have your 100th birthday.
Triennial, from Latin "triennium".