Count time from registration date to today

Just gave an example of how to do it:

$today_date      = new DateTime( date( 'Y-m-d', strtotime( 'today' ) ) );
    $register_date  = get_the_author_meta( 'user_registered', get_current_user_id() );
    $registered = new DateTime( date( 'Y-m-d', strtotime( $register_date ) ) );
    $interval_date   = $today_date->diff( $registered );
        if( $interval_date->days < 31 ) {
            echo 'With us ' . $interval_date->format('%d days');
            }
        elseif( $interval_date->days < 365 ) {
            echo 'With us ' . $interval_date->format('%m months %d days');
            }
        elseif( $interval_date->days > 365 ) {
            echo 'With us ' . $interval_date->format('%y years %m month %d days');
            }