diff --git a/flang/runtime/CMakeLists.txt b/flang/runtime/CMakeLists.txt
--- a/flang/runtime/CMakeLists.txt
+++ b/flang/runtime/CMakeLists.txt
@@ -66,6 +66,7 @@
   stop.cpp
   sum.cpp
   terminator.cpp
+  time-intrinsic.cpp
   tools.cpp
   transformational.cpp
   type-code.cpp
diff --git a/flang/runtime/time-intrinsic.cpp b/flang/runtime/time-intrinsic.cpp
new file mode 100644
--- /dev/null
+++ b/flang/runtime/time-intrinsic.cpp
@@ -0,0 +1,32 @@
+//===-- runtime/time-intrinsic.cpp ----------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Implements time-related intrinsic subroutines.
+
+#include "time-intrinsic.h"
+
+#include <ctime>
+
+namespace Fortran::runtime {
+extern "C" {
+
+// CPU_TIME (Fortran 2018 16.9.57)
+double RTNAME(CpuTime)() {
+  // This is part of the c++ standard, so it should at least exist everywhere.
+  // It probably does not have the best resolution, so we prefer other
+  // platform-specific alternatives if they exist.
+  std::clock_t timestamp{std::clock()};
+  if (timestamp != std::clock_t{-1}) {
+    return static_cast<double>(timestamp) / CLOCKS_PER_SEC;
+  }
+
+  // Return some negative value to represent failure.
+  return -1.0;
+}
+} // extern "C"
+} // namespace Fortran::runtime
diff --git a/flang/unittests/RuntimeGTest/CMakeLists.txt b/flang/unittests/RuntimeGTest/CMakeLists.txt
--- a/flang/unittests/RuntimeGTest/CMakeLists.txt
+++ b/flang/unittests/RuntimeGTest/CMakeLists.txt
@@ -11,6 +11,7 @@
   Random.cpp
   Reduction.cpp
   RuntimeCrashTest.cpp
+  Time.cpp
   Transformational.cpp
 )
 
diff --git a/flang/unittests/RuntimeGTest/Time.cpp b/flang/unittests/RuntimeGTest/Time.cpp
new file mode 100644
--- /dev/null
+++ b/flang/unittests/RuntimeGTest/Time.cpp
@@ -0,0 +1,35 @@
+//===-- flang/unittests/RuntimeGTest/Time.cpp -----------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "../../runtime/time-intrinsic.h"
+
+using namespace Fortran::runtime;
+
+volatile int x = 0;
+
+void LookBusy() {
+  // We're trying to track actual processor time, so sleeping is not an option.
+  // Doing some writes to a volatile variable should do the trick.
+  for (int i = 0; i < (1 << 8); ++i) {
+    x = i;
+  }
+}
+
+TEST(TimeIntrinsics, CpuTime) {
+  // We can't really test that we get the "right" result for CPU_TIME, but we
+  // can have a smoke test to see that we get something reasonable on the
+  // platforms where we expect to support it.
+  double start = RTNAME(CpuTime)();
+  LookBusy();
+  double end = RTNAME(CpuTime)();
+
+  ASSERT_GE(start, 0.0);
+  ASSERT_GT(end, 0.0);
+  ASSERT_GT(end, start);
+}