Index: libcxx/src/support/win32/thread_win32.cpp
===================================================================
--- libcxx/src/support/win32/thread_win32.cpp
+++ libcxx/src/support/win32/thread_win32.cpp
@@ -246,10 +246,8 @@
 
 void __libcpp_thread_sleep_for(const chrono::nanoseconds& __ns)
 {
-  using namespace chrono;
-  // round-up to the nearest milisecond
-  milliseconds __ms =
-      duration_cast<milliseconds>(__ns + chrono::nanoseconds(999999));
+  // round-up to the nearest millisecond
+  chrono::milliseconds __ms = std::chrono::ceil<chrono::milliseconds>(__ns);
   // FIXME(compnerd) this should be an alertable sleep (WFSO or SleepEx)
   Sleep(__ms.count());
 }