Fix #644: Only look at child Text nodes in <time>

Using textContent is more complicated or expensive. The concept of
looking at only child Text nodes is also used by, e.g., <title>.

source

@@ -19064,8 +19064,10 @@ this specification: the <abbr>WHATWG</abbr> and the
   descendants.</p>
 
   <p>The <dfn>datetime value</dfn> of a <code>time</code> element is the value of the element's
-  <code data-x="attr-time-datetime">datetime</code> content attribute, if it has one, or the
-  element's <code>textContent</code>, if it does not.</p>
+  <code data-x="attr-time-datetime">datetime</code> content attribute, if it has one, otherwise the
+  concatenation of the contents of all the <code>Text</code> nodes that are children of the
+  <code>time</code> element (ignoring any other nodes such as comments or elements), in <span>tree
+  order</span>.</p>
 
   <p>The <span>datetime value</span> of a <code>time</code> element must match one of the following
   syntaxes.</p>