Date before 1970: timestamp is stored as positive

Hi!

I'm having a small issue with storing pre-1970 dates as timestamp: they should be negative, but after saving the post the value is turned into the positive value.

So 1914-10-10 (-1740356372000) becomes 2025-02-24 (1740356372000) after saving.

Hope you can find a fix, so I can keep using your perfect plugin on the WW-I project I'm working on....

Best,

Aart Jan

 add_filter('rwmb_meta_boxes', function ($metaBoxes) {
    $metaBoxes[] = [
       'title'           => 'Gegevens',
       'style'             => 'seamless',
       'id'              => 'tijdlijn',
       'post_types'      => 'gebeurtenis',
       'fields'          => [
           [
               'id' => 'van',
               'type' => 'date',
               'admin_columns' => ['position'=> 'after title','sort'=>true],
               'name' => 'Vanaf',
               'timestamp' => true,
           ],
        ],
    ];
    return $metaBoxes;
});