util: add literal operators for duration types

Change-Id: I007fa61c80e535c6e4b12e85a32eeeee62fecc3c
Refs: #4468
diff --git a/src/security/v2/certificate-cache.cpp b/src/security/v2/certificate-cache.cpp
index be2b084..9726b7e 100644
--- a/src/security/v2/certificate-cache.cpp
+++ b/src/security/v2/certificate-cache.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).
  *
@@ -28,11 +28,10 @@
 
 NDN_LOG_INIT(ndn.security.v2.CertificateCache);
 
-const time::nanoseconds&
+time::nanoseconds
 CertificateCache::getDefaultLifetime()
 {
-  static time::nanoseconds lifetime = time::seconds(3600);
-  return lifetime;
+  return 1_h;
 }
 
 CertificateCache::CertificateCache(const time::nanoseconds& maxLifetime)