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_zos.h
   support/ibm/limits.h
   support/ibm/locale_mgmt_aix.h
   support/ibm/nanosleep.h
diff --git a/libcxx/include/support/ibm/gettod_zos.h b/libcxx/include/support/ibm/gettod_zos.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/support/ibm/gettod_zos.h
@@ -0,0 +1,55 @@
+// -*- 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>
+
+static inline
+int gettimeofdayMonotonic(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 = EMVSTODNOTSET;
+    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 DivPair0 = 0;  // dividend (upper half), remainder
+  uint64_t DivPair1 = us; // dividend (lower half), quotient
+  uint64_t Divisor = 1000000;
+  asm(" dlgr %0,%2"
+      : "+NR:r0"(DivPair0), "+NR:r1"(DivPair1)
+      : "r"(Divisor)
+      :);
+
+  Output->tv_sec = DivPair1;
+  Output->tv_nsec = DivPair0 * 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
@@ -9,6 +9,11 @@
 #include "chrono"
 #include "cerrno"        // errno
 #include "system_error"  // __throw_system_error
+
+#if defined(__MVS__)
+#include <support/ibm/gettod_zos.h> // gettimeofdayMonotonic
+#endif
+
 #include <time.h>        // clock_gettime and CLOCK_{MONOTONIC,REALTIME,MONOTONIC_RAW}
 #include "include/apple_availability.h"
 
@@ -159,6 +164,16 @@
   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 != gettimeofdayMonotonic(&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() {