diff --git a/projects/libcxx/src/chrono.cpp b/projects/libcxx/src/chrono.cpp
index f0a5d50..c7fbd90 100644
--- a/projects/libcxx/src/chrono.cpp
+++ b/projects/libcxx/src/chrono.cpp
@@ -33,6 +33,11 @@
 #  endif // !defined(CLOCK_REALTIME)
 #endif // defined(_LIBCPP_WIN32API)
 
+ // restore previous libcxx fallback
+#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200
+#include <mach/mach_time.h>
+#endif
+
 #if defined(__ELF__) && defined(_LIBCPP_LINK_RT_LIB)
 #  pragma comment(lib, "rt")
 #endif
@@ -110,6 +115,8 @@ const bool steady_clock::is_steady;
 
 #if defined(__APPLE__)
 
+#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ >= 101200
+
 #if !defined(CLOCK_MONOTONIC_RAW)
 #  error "Building libc++ on Apple platforms requires CLOCK_MONOTONIC_RAW"
 #endif
@@ -127,6 +134,61 @@ steady_clock::now() _NOEXCEPT
         __throw_system_error(errno, "clock_gettime(CLOCK_MONOTONIC_RAW) failed");
     return time_point(seconds(tp.tv_sec) + nanoseconds(tp.tv_nsec));
 }
+#else
+ // restore previous libcxx fallback
+ //
+ //   mach_absolute_time() * MachInfo.numer / MachInfo.denom is the number of
+ //   nanoseconds since the computer booted up.  MachInfo.numer and MachInfo.denom
+ //   are run time constants supplied by the OS.  This clock has no relationship
+ //   to the Gregorian calendar.  It's main use is as a high resolution timer.
+
+ // MachInfo.numer / MachInfo.denom is often 1 on the latest equipment.  Specialize
+ //   for that case as an optimization.
+
+ static
+ steady_clock::rep
+ steady_simplified()
+ {
+     return static_cast<steady_clock::rep>(mach_absolute_time());
+ }
+
+ static
+ double
+ compute_steady_factor()
+ {
+     mach_timebase_info_data_t MachInfo;
+     mach_timebase_info(&MachInfo);
+     return static_cast<double>(MachInfo.numer) / MachInfo.denom;
+ }
+
+ static
+ steady_clock::rep
+ steady_full()
+ {
+     static const double factor = compute_steady_factor();
+     return static_cast<steady_clock::rep>(mach_absolute_time() * factor);
+ }
+
+ typedef steady_clock::rep (*FP)();
+
+ static
+ FP
+ init_steady_clock()
+ {
+     mach_timebase_info_data_t MachInfo;
+     mach_timebase_info(&MachInfo);
+     if (MachInfo.numer == MachInfo.denom)
+         return &steady_simplified;
+     return &steady_full;
+ }
+
+ steady_clock::time_point
+ steady_clock::now() _NOEXCEPT
+ {
+     static FP fp = init_steady_clock();
+     return time_point(duration(fp()));
+ }
+#endif // restore previous libcxx fallback
 
 #elif defined(_LIBCPP_WIN32API)