<form action="" method="POST" class="myForm" autocomplete="off">
    <input id="dateChange" type="text" name="input-test" value="<?php echo str_replace('_', '', $jikuDate); ?>"  autocomplete="off">
    <input id="test-form" type="submit" name="updateDate" value="Update">
</form>

<?php

    if(isset($_POST['updateDate'])){
       $post = array(
          'ID' => $id
       );
       if ('Update' === ($_POST['updateDate'] ?? false)) {
         update_post_meta( $post->ID, 'usp-custom-14', $_POST['input-test']);
       }
   }
?>