Fix: Several fixes into date functions

htdocs/admin/ihm.php

htdocs/core/lib/date.lib.php

htdocs/holiday/class/holiday.class.php

htdocs/holiday/fiche.php

htdocs/holiday/index.php

htdocs/holiday/month_report.php

htdocs/install/mysql/tables/llx_holiday.sql

htdocs/install/mysql/tables/llx_holiday_users.sql

htdocs/langs/en_US/holiday.lang

htdocs/langs/fr_FR/holiday.lang

test/phpunit/DateLibTest.php