wpalchemy and datetime input

I'm trying to get to work a datetime input in a metabox with wpalchemy. The way I'm doing, I can get the date and time, but I need to pass this content, to a datetime object, using strtotime. And for this, I'm using the save_filter option. The function:

function save_datetime($meta, $post_id) {
    // var_dump($meta);
     //var_dump($post_id);
     //exit;
 
     $dd = $meta['start_day'];
     $mm = $meta['start_month'];
     $yy = $meta['start_year'];
     $hh = $meta['start_hour'];
     $mn = $meta['start_minute'];
     $start_datetime = strtotime( $dd . $mm . $yy . $hh . $mn );
 
     //$meta['start_datetime'] = $start_datetime;
     update_post_meta($post_id, '_info_evento_start_datetime', $start_datetime);
 
    return $meta;
 }
 

But the information is not recorded at the database. The two files can be found here and here.

How can I write in database with this function?

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