Schedules core, theme, and plugin update checks.

function wp_schedule_update_checks() {
	if ( ! wp_next_scheduled( 'wp_version_check' ) && ! wp_installing() ) {
		wp_schedule_event( time(), 'twicedaily', 'wp_version_check' );
	}

	if ( ! wp_next_scheduled( 'wp_update_plugins' ) && ! wp_installing() ) {
		wp_schedule_event( time(), 'twicedaily', 'wp_update_plugins' );
	}

	if ( ! wp_next_scheduled( 'wp_update_themes' ) && ! wp_installing() ) {
		wp_schedule_event( time(), 'twicedaily', 'wp_update_themes' );
	}
}