Include last seen in device boxes on graphviz map (#1633)

* Include last seen in device boxes on graphviz map

* Refactor

* Fix test cases

* Bind

* Refactor last seen label construction

* style fix

lib/util/utils.js

@@ -100,6 +100,27 @@ function getZigbee2mqttVersion(callback) {
     });
 }
 
+function formatDate(date, type, _default) {
+    let result;
+
+    switch (type) {
+    case 'ISO_8601':
+        result = new Date(date).toISOString();
+        break;
+    case 'ISO_8601_local':
+        result = toLocalISOString(new Date(date));
+        break;
+    case 'epoch':
+        result = date;
+        break;
+    default:
+        result = _default;
+        break;
+    }
+
+    return result;
+}
+
 module.exports = {
     millisecondsToSeconds: (milliseconds) => milliseconds / 1000,
     secondsToMilliseconds: (seconds) => seconds * 1000,
@@ -113,6 +134,7 @@ module.exports = {
     isNumeric: (string) => /^\d+$/.test(string),
     toLocalISOString: (dDate) => toLocalISOString(dDate),
     getPostfixes: () => postfixes,
+    formatDate: (date, type, _default=null) => formatDate(date, type, _default),
     correctDeviceType: (device) => {
         if (device) {
             if (forceEndDevice.includes(device.modelId)) {