Replace "Date" placeholder in classic header with year-month

add_filter( 'tribe-events-bar-filters', 'change_tribe_date_placeholder', 50 );

function change_tribe_date_placeholder( array $filters ) {
    $find = 'placeholder="Date"';
    $replace = 'placeholder="' . date('Y-m') . '"';

    if ( isset($filters['tribe-bar-date']) )
        $filters['tribe-bar-date']['html'] = str_replace($find, $replace, $filters['tribe-bar-date']['html']);

    return $filters;
}