diff --git a/src/walltime.cc b/src/walltime.cc
index da6d633e..4580c392 100644
--- a/src/walltime.cc
+++ b/src/walltime.cc
@@ -207,6 +207,8 @@ WallTime Now()
 }  // end namespace walltime
 
 
+namespace {
+
 std::string DateTimeString(bool local) {
   typedef std::chrono::system_clock Clock;
   std::time_t now = Clock::to_time_t(Clock::now());
@@ -225,4 +227,10 @@ std::string DateTimeString(bool local) {
   return std::string(storage);
 }
 
+} // end namespace
+
+std::string LocalDateTimeString() {
+  return DateTimeString(true);
+}
+
 }  // end namespace benchmark
diff --git a/src/walltime.h b/src/walltime.h
index face158c..38c26f33 100644
--- a/src/walltime.h
+++ b/src/walltime.h
@@ -10,11 +10,7 @@ namespace walltime {
 WallTime Now();
 }  // end namespace walltime
 
-std::string DateTimeString(bool local = false);
-
-inline std::string LocalDateTimeString() {
-    return DateTimeString(true);
-}
+std::string LocalDateTimeString();
 
 }  // end namespace benchmark