Add map last updated date to page

index.html

@@ -75,7 +75,8 @@
               View the server map
             </a>
             <p class="small-text">
-              Map is updated every day around 3 am EST.
+              Map is updated every day around 3 am EST. Last updated:&nbsp;
+              <span id="map-last-updated" class="small-text"></span>
             </p>
 
             <h2>Server Log</h2>
@@ -94,5 +95,6 @@
         <script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.19.2/moment.min.js"></script>
         <script src="js/getStatus.js"></script>
         <script src="js/getLog.js"></script>
+        <script src="js/getMapUpdate.js"></script>
     </body>
 </html>

js/getMapUpdate.js

@@ -0,0 +1,10 @@
+var mapUpdateTime = document.getElementById('map-last-updated');
+
+function getHeaderTime () {
+  mapUpdateTime.textContent = moment(this.getResponseHeader("Last-Modified")).calendar();
+}
+
+var oReq = new XMLHttpRequest();
+oReq.open("HEAD", "/map/index.html");
+oReq.onload = getHeaderTime;
+oReq.send();