src/logger: use unix timestamp as default

refs: #4187

Change-Id: I235c1ba60d981b7c52d50b14b6fefafca049679a
diff --git a/src/logger.cpp b/src/logger.cpp
index 0fb0bd3..580fc3f 100644
--- a/src/logger.cpp
+++ b/src/logger.cpp
@@ -55,7 +55,7 @@
   }
 
   log4cxx::PatternLayoutPtr
-           layout(new log4cxx::PatternLayout("%date{yyyyMMddHHmmssSSS} %p: [%c] %m%n"));
+           layout(new log4cxx::PatternLayout("%date{%s}.%date{SSS} %p: [%c] %m%n"));
 
   log4cxx::RollingFileAppender* rollingFileAppender =
            new log4cxx::RollingFileAppender(layout, logDir+"/nlsr.log", true);
@@ -85,4 +85,4 @@
          boost::iequals(logLevel, "debug") || boost::iequals(logLevel, "info")  ||
          boost::iequals(logLevel, "warn")  || boost::iequals(logLevel, "error") ||
          boost::iequals(logLevel, "none");
-}
\ No newline at end of file
+}