src/lsa-segment-storage: fix convergence w/o content store

refs: #4430

Change-Id: I5124e9fd49bd9850c904c31d221b42b4249553a3
diff --git a/src/lsa-segment-storage.cpp b/src/lsa-segment-storage.cpp
index 3e85bd0..c384a53 100644
--- a/src/lsa-segment-storage.cpp
+++ b/src/lsa-segment-storage.cpp
@@ -107,7 +107,7 @@
 
     try {
       expirationTime = ndn::time::duration_cast<ndn::time::seconds>
-                       (ndn::time::system_clock::now() - ndn::time::fromIsoString(options.at(3)));
+                       (ndn::time::fromIsoString(options.at(3)) - ndn::time::system_clock::now());
     } catch (const std::exception& e) {
       NLSR_LOG_ERROR("Cannot extract expiration time from LSA content: " << e.what());
     }