Normalize month-mday before finding epoch

	Especially over the year 2038, 30 Feb and so on may cause odd behavior
	on validating found epoch with given year-month-day [Bug #15340]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@66967 b2dd03c8-39d4-4d8f-98ff-823fe69b080e