wpdb insert formatting for date in array

I have this simple wpdb->insert:

$this->wpdb->insert( 
'tablename',
 array(...., field1 => 'blahblah' , 'birthdate' => (isset($this->dob)) ? esc_attr(date('Y-m-d', strtotime($this->dob))) : null, ...) , 
array( ..... , '%s', date('Y-m-d'), .... ) 
); 

to set format for date in the second array what I should put for wpdb?
Thanks in advance!
Cheers! 🙂