diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 960bae3..c4cde86 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -1158,6 +1158,11 @@ dnl --enable-libstdcxx-time=no
 dnl --disable-libstdcxx-time
 dnl        disables the checks completely
 dnl
+dnl N.B. Darwin provides nanosleep but doesn't support the whole POSIX
+dnl Timers option, so doesn't define _POSIX_TIMERS. Because the test
+dnl below fails Darwin unconditionally defines _GLIBCXX_USE_NANOSLEEP in
+dnl os_defines.h and also defines _GLIBCXX_USE_SCHED_YIELD.
+dnl
 AC_DEFUN([GLIBCXX_ENABLE_LIBSTDCXX_TIME], [
 
   AC_MSG_CHECKING([for clock_gettime, nanosleep and sched_yield])
diff --git a/libstdc++-v3/config/os/bsd/darwin/os_defines.h b/libstdc++-v3/config/os/bsd/darwin/os_defines.h
index 421478d..be4aa14 100644
--- a/libstdc++-v3/config/os/bsd/darwin/os_defines.h
+++ b/libstdc++-v3/config/os/bsd/darwin/os_defines.h
@@ -42,4 +42,9 @@
 // Static initializer macro is buggy in darwin, see libstdc++/51906
 #define _GTHREAD_USE_RECURSIVE_MUTEX_INIT_FUNC
 
+// Configure checks for nanosleep fail on Darwin, but nanosleep and
+// sched_yield are always available, so use them.
+#define _GLIBCXX_USE_NANOSLEEP 1
+#define _GLIBCXX_USE_SCHED_YIELD 1
+
 #endif