use hrtime to calculate timestamp

lib/RequestLogger.js

@@ -203,6 +203,8 @@ class RequestLogger {
 
         this.startTime = process.hrtime();
         this.elapsedTime = null;
+
+        this._timestamp = Date.now();
     }
 
     /**
@@ -434,6 +436,7 @@ class RequestLogger {
                 });
             return;
         }
+        let eMs;
         const fields = objectCopy({}, this.fields, logFields || {});
         const endFlag = isEnd || false;
 
@@ -442,7 +445,8 @@ class RequestLogger {
         * uses this field to generate ISO 8601 timestamp
         */
         if (fields.time === undefined) {
-            fields.time = Date.now();
+            eMs = process.hrtime(this.startTime);
+            fields.time = this._timestamp + ( eMs[0] * 1000 + eMs[1] / 1000000);
         }
 
         fields.req_id = serializeUids(this.uids);