browse: Serve a datetime format the IE11 and Firefox understand

Another new safeguard is that we check whether the datetime has been read
correctly. If not then the listing will not be localized.

closes #793

caddy/setup/browse.go

@@ -385,7 +385,7 @@ footer {
 						{{- else}}
 						<td data-order="{{.Size}}">{{.HumanSize}}</td>
 						{{- end}}
-						<td class="hideable"><time datetime="{{.HumanModTime "2006-01-02 15:04:05-0700"}}">{{.HumanModTime "01/02/2006 03:04:05 PM"}}</time></td>
+						<td class="hideable"><time datetime="{{.HumanModTime "2006-01-02T15:04:05Z"}}">{{.HumanModTime "01/02/2006 03:04:05 PM -07:00"}}</time></td>
 					</tr>
 					{{- end}}
 					</tbody>
@@ -401,6 +401,12 @@ footer {
 					return;
 				}
 				var d = new Date(e.getAttribute('datetime'));
+				if (isNaN(d)) {
+					d = new Date(e.textContent);
+					if (isNaN(d)) {
+						return;
+					}
+				}
 				e.textContent = d.toLocaleString();
 			}
 			var timeList = Array.prototype.slice.call(document.getElementsByTagName("time"));