Testing confirms many leap years return 0, "not leap"; some non-leap years return 1, "are leap", some return correct results. Same results in version 5.1.4.2 build ID 1:5.1.4-0ubunto1 and version 4.4.3.2 Build ID: 40m0(Build:2) Range years tested 0 - 3000.
Please be more specific, which cases give a wrong result and why you think, that the result is wrong.
Created attachment 130310 [details] Known correct determinations of leap years vs isleapyear() function Sequence of 3001 years tested in 4 known correct methods of determining leap year are used as arguments to isleapyear() function resulting in many false positives and many missed leap years.
A calc sheet would had been more usefull than a pdf. We can't see how you are trying to use this function. ISLEAPYEAR need a true date as argument, see the help for a correct use of the function : https://help.libreoffice.org/Calc/Add-in_Functions#ISLEAPYEAR
What is needed?
(In reply to Louie from comment #4) > What is needed? As said in comment 3, could you please attach a calc sheet ?
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This was indeed NOTABUG. As GerardF mentioned in comment 3, ISLEAPYEAR expects a *date* as its argument, while Louie used calculation like '=ISLEAPYEAR(2021)' to learn is 2021 was a leap year. The correct calculation would be e.g. '=ISLEAPYEAR("2021-01-01")'.