Add time format with client side date parsing

composer.json

@@ -28,7 +28,7 @@
 		"wp-coding-standards/wpcs": "2.*",
 		"wp-phpunit/wp-phpunit": "~6.0",
 		"wporg/wporg-repo-tools": "dev-trunk",
-		"yoast/phpunit-polyfills": "^1.0"
+		"yoast/phpunit-polyfills": "^1.1"
 	},
 	"config": {
 		"allow-plugins": {

mu-plugins/blocks/time/index.php

@@ -0,0 +1,65 @@
+<?php
+/**
+ * Format Name: Time
+ * Description: Attempts to parse a time string like 'Tuesday, April 5th, at 15:00 UTC' relative to the post date,
+ * and creates a format that shows it in the viewer's local time zone.
+ *
+ * @package wporg
+ */
+
+namespace WordPressdotorg\MU_Plugins\Time;
+
+add_action( 'init', __NAMESPACE__ . '\init' );
+
+/**
+ * Enqueues the scripts for the Time format in the block editor and frontend using the metadata loaded from the `block.json` file.
+ */
+function init() {
+	// Add the JS script to add the Time formatting option to the toolbar. The dependencies are autogenerated in block.json,
+	// and can be read with `wp_json_file_decode` & `register_block_script_handle.
+	$metadata_file = __DIR__ . '/build/block.json';
+	$metadata = wp_json_file_decode( $metadata_file, array( 'associative' => true ) );
+	$metadata['file'] = $metadata_file;
+
+	$editor_script_handle = register_block_script_handle( $metadata, 'editorScript' );
+	add_action(
+		'enqueue_block_assets',
+		function() use ( $editor_script_handle ) {
+			if ( wp_should_load_block_editor_scripts_and_styles() && is_admin() ) {
+				wp_enqueue_script( $editor_script_handle );
+			}
+		}
+	);
+
+	$view_script_handle = register_block_script_handle( $metadata, 'viewScript' );
+	add_action(
+		'wp_enqueue_scripts',
+		function() use ( $view_script_handle ) {
+			if ( ! is_admin() ) {
+				wp_enqueue_script( $view_script_handle );
+				wp_script_add_data( $view_script_handle, 'group', 1 );
+			}
+		}
+	);
+
+	add_filter( 'wp_kses_allowed_html', __NAMESPACE__ . '\add_time_to_allowed_html', 10, 2 );
+}
+
+/**
+ * Adds the `time` tag to the allowed HTML tags for the block editor.
+ *
+ * @param array  $allowed_tags Allowed tags, attributes, and/or entities.
+ * @param string $context      Context to judge allowed tags by.
+ * @return array Allowed tags, attributes, and/or entities.
+ */
+function add_time_to_allowed_html( $allowed_tags, $context ) {
+	if ( 'post' === $context ) {
+		$allowed_tags['time'] = array(
+			'class'    => true,
+			'data-iso' => true,
+			'datetime' => true,
+		);
+	}
+
+	return $allowed_tags;
+}