diff --git a/include/c11log/sinks/file_sinks.h b/include/c11log/sinks/file_sinks.h
index 798c7c4a..d1a140be 100644
--- a/include/c11log/sinks/file_sinks.h
+++ b/include/c11log/sinks/file_sinks.h
@@ -149,11 +149,10 @@ private:
 
     static std::string _calc_filename(const std::string& basename, const std::string& extension)
     {
-        std::ostringstream oss;
         std::tm tm = c11log::details::os::localtime();
         char buf[32];
-	    sprintf(buf, ".%Y-%m-%d.", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday);
-        return buf;
+	    sprintf(buf, ".%d-%02d-%02d.", tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday);
+        return basename+buf;
     }
 
     std::string _base_filename;