Docs: Refresh restricted datetime functions docs

Kept the descriptions for individual code comparisons simpler, and populated the <standard> element with more description instead.

WordPress/Docs/DateTime/RestrictedFunctionsStandard.xml

@@ -0,0 +1,44 @@
+<documentation title="Restricted Date and Time Functions">
+    <standard>
+        <![CDATA[
+      The restricted functions date_default_timezone_set() and date() should not be used.
+
+      Using date_default_timezone_set() isn't allowed, because WordPress core needs default time zone set to UTC. Use DateTime object or WP core API functions instead.
+
+      Using date() isn't allowed, as it is affected by runtime timezone changes which can cause date/time to be incorrectly displayed. Use gmdate() instead.
+    ]]>
+    </standard>
+    <code_comparison>
+        <code title="Valid: Using DateTime object.">
+            <![CDATA[
+$date = new DateTime();
+$date->setTimezone(
+    new DateTimeZone( 'Europe/Amsterdam' )
+);
+        ]]>
+        </code>
+        <code title="Invalid: Using date_default_timezone_set().">
+            <![CDATA[
+date_default_timezone_set( 'Europe/Amsterdam' );
+        ]]>
+        </code>
+    </code_comparison>
+    <code_comparison>
+        <code title="Valid: Using gmdate().">
+            <![CDATA[
+$last_updated = gmdate(
+    'Y-m-d\TH:i:s',
+    strtotime( $plugin['last_updated'] )
+);
+        ]]>
+        </code>
+        <code title="Invalid: Using date().">
+            <![CDATA[
+$last_updated = date(
+    'Y-m-d\TH:i:s',
+    strtotime( $plugin['last_updated'] )
+);
+        ]]>
+        </code>
+    </code_comparison>
+</documentation>