util: add literal operators for duration types

Change-Id: I007fa61c80e535c6e4b12e85a32eeeee62fecc3c
Refs: #4468
diff --git a/tests/unit-tests/mgmt/status-dataset-context.t.cpp b/tests/unit-tests/mgmt/status-dataset-context.t.cpp
index 5c432a7..c5af199 100644
--- a/tests/unit-tests/mgmt/status-dataset-context.t.cpp
+++ b/tests/unit-tests/mgmt/status-dataset-context.t.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -54,7 +54,7 @@
               [this] (const ControlResponse& resp) {
                 sendNackHistory.push_back(resp);
               })
-    , defaultImsFresh(time::milliseconds(1000))
+    , defaultImsFresh(1000_ms)
   {
   }
 
@@ -141,8 +141,8 @@
 
 BOOST_AUTO_TEST_CASE(GetAndSet)
 {
-  auto period = time::milliseconds(9527);
-  BOOST_CHECK_EQUAL(context.getExpiry(), time::milliseconds(1000));
+  auto period = 9527_ms;
+  BOOST_CHECK_EQUAL(context.getExpiry(), 1000_ms);
   BOOST_CHECK_EQUAL(context.setExpiry(period).getExpiry(), period);
 }