Handle dates outside of strtotime()'s range in date_to_int, int_to_date

Close #9

tests/MD_STD_Test.php

@@ -27,8 +27,8 @@ public static function date_to_int_provider():array {
             ["-0022-01-01", -220101],
             ["-2022-01-01", -20220101],
             ["-0001-01-01", -10101],
-            ["-0000-01-01", -101],
             ["0000-01-01", 101],
+            ["100000-01-01", 1000000101],
         ];
 
         $output = [];