https://www.php.net/manual/zh/intlgregoriancalendar.isleapyear.php
PHP: IntlGregorianCalendar::isLeapYear - Manual
Determine if the given year is a leap year
phpmanual
https://learn.microsoft.com/pl-pl/dotnet/api/system.globalization.eastasianlunisolarcalendar.isleapyear?view=netframework-4.8.1&viewFallbackFrom=net-10.0
EastAsianLunisolarCalendar.IsLeapYear(Int32, Int32) Method (System.Globalization) | Microsoft Learn
Determines whether the specified year in the specified era is a leap year.
methodsystemglobalizationmicrosoftlearn
https://profoundlogicsupport.atlassian.net/wiki/spaces/PUI/pages/164498373/date.isLeapYear+method
date.isLeapYear() method - Profound Logic Documentation - Profound Logic
datemethodprofoundlogicdocumentation