<https://docs.ruby-lang.org/ja/latest/method/Date/s/gregorian_leap=3f.html> ```ruby irb(main):001> require 'date' => true irb(main):002> Date.leap?(2024) => true irb(main):003> Date.leap?(2025) => false ``` インスタンスメソッドもある <https://docs.ruby-lang.org/ja/latest/method/Date/i/leap=3f.html>