diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -530,6 +530,12 @@
     target_add_compile_flags_if_supported(${target} PRIVATE -faligned-allocation)
   endif()
 
+  # As part of monotonic clock support on z/OS we need macro _LARGE_TIME_API
+  # to be defined before any system header is included.
+  if (ZOS)
+    add_definitions(-D_LARGE_TIME_API) # Needed for struct timespec64.
+  endif()
+
   # On all systems the system c++ standard library headers need to be excluded.
   # MSVC only has -X, which disables all default includes; including the crt.
   # Thus, we do nothing and hope we don't accidentally include any of the C++
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -151,6 +151,7 @@
   strstream
   support/android/locale_bionic.h
   support/fuchsia/xlocale.h
+  support/ibm/gettod.h
   support/ibm/limits.h
   support/ibm/locale_mgmt_aix.h
   support/ibm/nanosleep.h
diff --git a/libcxx/include/support/ibm/gettod.h b/libcxx/include/support/ibm/gettod.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/support/ibm/gettod.h
@@ -0,0 +1,54 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_SUPPORT_IBM_GETTOD_H
+#define _LIBCPP_SUPPORT_IBM_GETTOD_H
+
+#include <time.h>
+
+inline int gettimeofday_monotonic(struct timespec64 *output) {
+
+   // The POSIX gettimeofday() function is not available on z/OS. Therefore,
+   // we will call stcke and other hardware instructions in implement equivalent.
+   // Note that nanoseconds alone will overflow when reaching new epoch in 2042.
+ 
+   struct _t {
+      uint64_t hi;
+      uint64_t lo;
+   };
+   struct _t value = {0, 0};
+   uint64_t cc = 0;
+   asm(" stcke %0\n"
+       " ipm %1\n"
+       " srlg %1,%1,28\n"
+       : "=m"(value), "+r"(cc)::);
+
+   if (cc != 0) {
+      errno = EPERM;
+      return cc;
+   }
+   uint64_t us = (value.hi >> 4);
+   uint64_t ns = ((value.hi & 0x0F) << 8) + (value.lo >> 56);
+   ns = (ns * 1000) >> 12;
+   us = us - 2208988800000000;
+
+   uint64_t div_pair0 = 0;
+   uint64_t div_pair1 = us;
+   uint64_t divisor = 1000000;
+   asm(" dlgr %0,%2"
+       : "+NR:r0"(div_pair0), "+NR:r1"(div_pair1)
+       : "r"(divisor)
+       :);
+
+   output->tv_sec = div_pair1;
+   output->tv_nsec = div_pair0 * 1000 + ns;
+   return 0;
+}
+
+#endif // _LIBCPP_SUPPORT_IBM_GETTOD_H
diff --git a/libcxx/src/chrono.cpp b/libcxx/src/chrono.cpp
--- a/libcxx/src/chrono.cpp
+++ b/libcxx/src/chrono.cpp
@@ -20,6 +20,10 @@
 # include <sys/time.h> // for gettimeofday and timeval
 #endif
 
+#if defined(__MVS__)
+# include <support/ibm/gettod.h>
+#endif
+
 #if !defined(__APPLE__) && _POSIX_TIMERS > 0
 # define _LIBCPP_USE_CLOCK_GETTIME
 #endif
@@ -159,6 +163,17 @@
   return steady_clock::time_point(steady_clock::duration(dur));
 }
 
+#elif defined(__MVS__)
+
+static steady_clock::time_point __libcpp_steady_clock_now() {
+  struct timespec64 ts;
+  if (0 != gettimeofday_monotonic(&ts))
+        __throw_system_error(errno, "failed to obtain time of day");
+
+  return steady_clock::time_point(seconds(ts.tv_sec) +
+                                  nanoseconds(ts.tv_nsec));
+}
+
 #elif defined(CLOCK_MONOTONIC)
 
 static steady_clock::time_point __libcpp_steady_clock_now() {