diff --git a/core/string.cpp b/core/string.cpp
index 82535619..747d4aef 100644
--- a/core/string.cpp
+++ b/core/string.cpp
@@ -29,8 +29,9 @@ const std::string string :: sec2str(unsigned int sec)
 	return ss.str();
 }
 
-const std::string _time_detail(unsigned int value, unsigned int remaining,
-				const std::string &field)
+static const std::string _time_detail(unsigned int value,
+				      unsigned int remaining,
+				      const std::string &field)
 {
 	std::string res;