--- a/src/PatternLayout.cpp
+++ b/src/PatternLayout.cpp
@@ -24,6 +24,7 @@
 #include <iomanip>
 #include <ctime>
 #include <cstdlib>
+#include <cmath>
 #include "Localtime.hh"
 
 #ifdef LOG4CPP_HAVE_INT64_T