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Find Leap Years

By using LeapYearQ with DateDifference, you can find leap years and the intervals between them.

    
Find the leap years that occur from 1600 to 2012:
Wolfram Language code: leapyears = Select[DateRange[{1600}, {2012}, "Year"], LeapYearQ[#]&]
Find the intervals between them:
Wolfram Language code: Table[DateDifference[leapyears[[i]], leapyears[[i + 1]], "Year"], {i, 1, Length[leapyears] - 1}]//Union
While leap years most commonly come in four-year intervals, they sometimes come every eight years. This is because a traditional leap day every four years is actually a slight overcompensation in the calendar. Thus, a leap year is skipped every 100 years when those years are not divisible by 400:
Wolfram Language code: Table[DateDifference[leapyears[[i]], leapyears[[i + 1]], "Year"], {i, 1, Length[leapyears] - 1}]//Union
Find the years where an eight-year interval between leap years exists:
Wolfram Language code: DeleteCases[Table[If[DateDifference[leapyears[[i]], leapyears[[i + 1]], "Year"] === {8, "Year"}, {leapyears[[i]], leapyears[[i + 1]]}], {i, 1, Length[leapyears] - 1}], Null]