Renders the `core/calendar` block on server.

function render_block_core_calendar( $attributes ) {
	global $monthnum, $year;

	// Calendar shouldn't be rendered
	// when there are no published posts on the site.
	if ( ! block_core_calendar_has_published_posts() ) {
		if ( is_user_logged_in() ) {
			return '
' . __( 'The calendar block is hidden because there are no published posts.' ) . '
';
		}
		return '';
	}

	$previous_monthnum = $monthnum;
	$previous_year     = $year;

	if ( isset( $attributes['month'] ) && isset( $attributes['year'] ) ) {
		$permalink_structure = get_option( 'permalink_structure' );
		if (
			str_contains( $permalink_structure, '%monthnum%' ) &&
			str_contains( $permalink_structure, '%year%' )
		) {
			// phpcs:ignore WordPress.WP.GlobalVariablesOverride.OverrideProhibited
			$monthnum = $attributes['month'];
			// phpcs:ignore WordPress.WP.GlobalVariablesOverride.OverrideProhibited
			$year = $attributes['year'];
		}
	}

	$wrapper_attributes = get_block_wrapper_attributes();
	$output             = sprintf(
		'
%2$s
',
		$wrapper_attributes,
		get_calendar( true, false )
	);

	// phpcs:ignore WordPress.WP.GlobalVariablesOverride.OverrideProhibited
	$monthnum = $previous_monthnum;
	// phpcs:ignore WordPress.WP.GlobalVariablesOverride.OverrideProhibited
	$year = $previous_year;

	return $output;
}