Hello @Sifa Tok 

You can write a Catalog Client Script :-

var openedDate = g_form.getValue('opened_date');
var days = g_form.getValue('how_long_server...');
openedDate.addDays(days);
g_form.setValue('server_offline_until',openedDate);

Plz Mark my Solution as Accept and Give me thumbs up, if you find it helpful.

Regards,

Samaksh