In 1705, English astronomer Edmond Halley was the first to recognize comet appearances as periodic. The comet named after him, Halley's Comet, appears every 75-76 years! Last time by was 1986, so we won't see it again until 2061... unless it's cloudy that night...
Well, meet Derik's Comet!
Ok, I made it up, but it will help us understand how to use the YEAR() function to breakout the year from a date in Excel.
Easy! Now if we want to find the number of years between comet appearances, we just subtract the previous year from the latest year.
Vwalah! Not exactly periodic, but hey, maybe Sir Isaac Newton's laws of gravitation don't apply in blogs?