diff --git a/include/toml11/fwd/datetime_fwd.hpp b/include/toml11/fwd/datetime_fwd.hpp
index 81bdf17..44616a1 100644
--- a/include/toml11/fwd/datetime_fwd.hpp
+++ b/include/toml11/fwd/datetime_fwd.hpp
@@ -12,17 +12,6 @@
 namespace toml
 {
 
-// To avoid non-threadsafe std::localtime. In C11 (not C++11!), localtime_s is
-// provided in the absolutely same purpose, but C++11 is actually not compatible
-// with C11. We need to dispatch the function depending on the OS.
-namespace detail
-{
-std::tm localtime_s(const std::time_t* src);
-std::tm gmtime_s(const std::time_t* src);
-} // detail
-
-// ----------------------------------------------------------------------------
-
 enum class month_t : std::uint8_t
 {
     Jan =  0,
diff --git a/tests/test_find.cpp b/tests/test_find.cpp
index 385b65b..976eede 100644
--- a/tests/test_find.cpp
+++ b/tests/test_find.cpp
@@ -17,6 +17,15 @@
 #include <string_view>
 #endif
 
+namespace toml
+{
+namespace detail
+{
+std::tm localtime_s(const std::time_t* src);
+std::tm gmtime_s(const std::time_t* src);
+} // detail
+} // toml
+
 TEST_CASE("testing toml::find with toml type")
 {
     using value_type = toml::value;
diff --git a/tests/test_get.cpp b/tests/test_get.cpp
index ed46faf..ba8e15e 100644
--- a/tests/test_get.cpp
+++ b/tests/test_get.cpp
@@ -17,6 +17,15 @@
 #include <string_view>
 #endif
 
+namespace toml
+{
+namespace detail
+{
+std::tm localtime_s(const std::time_t* src);
+std::tm gmtime_s(const std::time_t* src);
+} // detail
+} // toml
+
 TEST_CASE("testing toml::get with toml types")
 {
     using value_type = toml::value;