diff --git a/src/security/certificate-cache-ttl.cpp b/src/security/certificate-cache-ttl.cpp
index 6f831d0..0997182 100644
--- a/src/security/certificate-cache-ttl.cpp
+++ b/src/security/certificate-cache-ttl.cpp
@@ -17,32 +17,34 @@
 
 namespace ndn {
 
-CertificateCacheTtl::CertificateCacheTtl(shared_ptr<boost::asio::io_service> io, int defaultTtl)
+CertificateCacheTtl::CertificateCacheTtl(shared_ptr<boost::asio::io_service> io, const time::seconds& defaultTtl)
   : m_defaultTtl(defaultTtl)
   , m_scheduler(*io)
 {}
 
 CertificateCacheTtl::~CertificateCacheTtl()
 {}
-  
+
 void
 CertificateCacheTtl::insertCertificate(ptr_lib::shared_ptr<const IdentityCertificate> certificate)
 {
-  time::Duration expire = (certificate->getFreshnessPeriod() >= 0 ? time::milliseconds(certificate->getFreshnessPeriod()) : time::seconds(m_defaultTtl));
+  time::milliseconds expire = (certificate->getFreshnessPeriod() >= time::seconds::zero() ?
+                         certificate->getFreshnessPeriod() : m_defaultTtl);
 
   Name trackerIndex = certificate->getName().getPrefix(-1);
   EventTracker::iterator it = m_tracker.find(trackerIndex);
   if(it != m_tracker.end())
       m_scheduler.cancelEvent(m_tracker[trackerIndex]);
 
-  m_scheduler.scheduleEvent(time::seconds(0), bind(&CertificateCacheTtl::insert, this, certificate));  
-  m_tracker[trackerIndex] = m_scheduler.scheduleEvent(expire, bind(&CertificateCacheTtl::remove, this, certificate->getName()));
+  m_scheduler.scheduleEvent(time::seconds(0), bind(&CertificateCacheTtl::insert, this, certificate));
+  m_tracker[trackerIndex] = m_scheduler.scheduleEvent(expire, bind(&CertificateCacheTtl::remove,
+                                                                   this, certificate->getName()));
 
 }
 
 void
 CertificateCacheTtl::insert(ptr_lib::shared_ptr<const IdentityCertificate> certificate)
-{ 
+{
   Name name = certificate->getName().getPrefix(-1);
   m_cache[name] = certificate;
 }
@@ -56,7 +58,7 @@
     m_cache.erase(it);
 }
 
-ptr_lib::shared_ptr<const IdentityCertificate> 
+ptr_lib::shared_ptr<const IdentityCertificate>
 CertificateCacheTtl::getCertificate(const Name & certificateName)
 {
   Cache::iterator it = m_cache.find(certificateName);
@@ -67,5 +69,3 @@
 }
 
 } // namespace ndn
-
-