https://bugs.php.net/bug.php?id=61135
PHP :: Bug #61135 :: gregoriantojd() returns incorrect date for some dates
phpbugreturnsincorrectdate
https://www.php.net/manual/it/function.gregoriantojd.php
PHP: GregorianToJD - Manual
Converte una data Gregoriana in un Giorno Giuliano
phpmanual