Use ISO8601 date format

The manually formatted date string was very close to
ISO8601 already, so just it directly.
Conforms to standards, is much easier in code (less manual work),
and carries timezone information.

tinystatus

@@ -108,7 +108,7 @@ for file in ${tmp}/ok/*.status; do
 done
 cat << EOF
 </ul>
-<p class=small> Last check: $(date +"%Y/%m/%d %H:%M:%S")</p>
+<p class=small> Last check: $(date -I'seconds')</p>
 EOF
 if [ -f "${incidentsfile}" ]; then
     echo '<h1>Incidents</h1>'