REST API: add a utility function, `mysql_to_rfc3339()` to `functions.php`

Background:
6d0ad766ca

Props rmmcue.
See #33982.

Built from https://develop.svn.wordpress.org/trunk@34846


git-svn-id: http://core.svn.wordpress.org/trunk@34811 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -5022,3 +5022,21 @@ function wp_post_preview_js() {
 	</script>
 	<?php
 }
+
+/**
+ * Parses and formats a MySQL datetime (Y-m-d H:i:s) for ISO8601/RFC3339.
+ *
+ * Explicitly strips timezones, as datetimes are not saved with any timezone
+ * information. Including any information on the offset could be misleading.
+ *
+ * @since 4.4.0
+ *
+ * @param string $date_string Date string to parse and format.
+ * @return string Date formatted for ISO8601/RFC3339.
+ */
+function mysql_to_rfc3339( $date_string ) {
+	$formatted = mysql2date( 'c', $date_string, false );
+
+	// Strip timezone information
+	return preg_replace( '/(?:Z|[+-]\d{2}(?::\d{2})?)$/', '', $formatted );
+}

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-alpha-34845';
+$wp_version = '4.4-alpha-34846';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.