Use monotonic clock for performance.now()

Author: Olivier Payen

React/CxxBridge/RCTJSIExecutorRuntimeInstaller.mm

@@ -22,8 +22,14 @@
     bindNativeLogger(runtime, iosLoggingBinder);
 
     PerformanceNow iosPerformanceNowBinder = []() {
-      auto time = std::chrono::system_clock::now().time_since_epoch();
-      return std::chrono::duration_cast<std::chrono::milliseconds>(time).count();
+      auto time = std::chrono::steady_clock::now();
+      auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(
+                          time.time_since_epoch())
+                          .count();
+
+      constexpr double NANOSECONDS_IN_MILLISECOND = 1000000.0;
+
+      return duration / NANOSECONDS_IN_MILLISECOND;
     };
     bindNativePerformanceNow(runtime, iosPerformanceNowBinder);
 

ReactAndroid/src/main/jni/react/jni/NativeTime.cpp

@@ -12,8 +12,14 @@ namespace facebook {
 namespace react {
 
 double reactAndroidNativePerformanceNowHook() {
-  auto time = std::chrono::system_clock::now().time_since_epoch();
-  return std::chrono::duration_cast<std::chrono::milliseconds>(time).count();
+  auto time = std::chrono::steady_clock::now();
+  auto duration = std::chrono::duration_cast<std::chrono::nanoseconds>(
+                      time.time_since_epoch())
+                      .count();
+
+  constexpr double NANOSECONDS_IN_MILLISECOND = 1000000.0;
+
+  return duration / NANOSECONDS_IN_MILLISECOND;
 }
 
 } // namespace react