diff --git a/flang-rt/lib/runtime/extensions.cpp b/flang-rt/lib/runtime/extensions.cpp
index e70dff3997233..f6c39468d5655 100644
--- a/flang-rt/lib/runtime/extensions.cpp
+++ b/flang-rt/lib/runtime/extensions.cpp
@@ -27,10 +27,7 @@
 #include <thread>
 
 #ifdef _WIN32
-#define WIN32_LEAN_AND_MEAN
-#define NOMINMAX
-#include <windows.h>
-
+#include "flang/Common/windows-include.h"
 #include <synchapi.h>
 
 inline void CtimeBuffer(char *buffer, size_t bufsize, const time_t cur_time,
@@ -309,6 +306,9 @@ void RTNAME(Perror)(const char *str) { perror(str); }
 // GNU extension function TIME()
 std::int64_t RTNAME(time)() { return time(nullptr); }
 
+// MCLOCK: returns accumulated CPU time in ticks
+std::int32_t FORTRAN_PROCEDURE_NAME(mclock)() { return std::clock(); }
+
 // Extension procedures related to I/O
 
 namespace io {
diff --git a/flang/docs/Intrinsics.md b/flang/docs/Intrinsics.md
index 0118f8eb7d913..f7da6c889d413 100644
--- a/flang/docs/Intrinsics.md
+++ b/flang/docs/Intrinsics.md
@@ -709,8 +709,9 @@ CACHESIZE, EOF, FP_CLASS, INT_PTR_KIND, ISNAN, LOC
 MALLOC, FREE
 ```
 
-### Library subroutine
+### Library subroutines and functions
 ```
+ticks = MCLOCK()
 CALL BACKTRACE()
 CALL FDATE(TIME)
 CALL GETLOG(USRNAME)
diff --git a/flang/include/flang/Runtime/extensions.h b/flang/include/flang/Runtime/extensions.h
index 06ae7f35d9b5b..b350204714431 100644
--- a/flang/include/flang/Runtime/extensions.h
+++ b/flang/include/flang/Runtime/extensions.h
@@ -12,14 +12,12 @@
 #ifndef FORTRAN_RUNTIME_EXTENSIONS_H_
 #define FORTRAN_RUNTIME_EXTENSIONS_H_
 
-#include "flang/Runtime/entry-names.h"
-
-#define FORTRAN_PROCEDURE_NAME(name) name##_
-
 #include "flang/Runtime/entry-names.h"
 #include <cstddef>
 #include <cstdint>
 
+#define FORTRAN_PROCEDURE_NAME(name) name##_
+
 #ifdef _WIN32
 // UID and GID don't exist on Windows, these exist to avoid errors.
 typedef std::uint32_t uid_t;
@@ -89,5 +87,8 @@ int FORTRAN_PROCEDURE_NAME(ierrno)();
 // GNU extension subroutine PERROR(STRING)
 void RTNAME(Perror)(const char *str);
 
+// MCLOCK -- returns accumulated time in ticks
+int FORTRAN_PROCEDURE_NAME(mclock)();
+
 } // extern "C"
 #endif // FORTRAN_RUNTIME_EXTENSIONS_H_