compare date between in meta_query

Hi,
on my page, the user chooses a event date. This date should be between the start and end date of a event.
How can i bring this query to work?

add_action('pre_get_posts', 'alter_query');

function alter_query($query) {

	if (is_search() && $query->query_vars['post_type'] == 'event') {

		if (!empty($_GET['selected_date'])) {

			$arr_selected_date = explode('-', $_GET['selected_date']);
			$selected_date = mktime(0, 0, 0, $arr_selected_date[1], $arr_selected_date[2], $arr_selected_date[0]) * 1000;

			$query->set(
				'meta_query',
				array(
					'relation' => 'AND',
					array(
						'key' => '_event-date-from',
						'value' => $selected_date,
						'compare' => '<='
						),
					array(
						'key' => '_event-date-to',
						'value' => $selected_date,
						'compare' => '>='
						)
					)
				);

		}

	}

}