From 5ac0640cbec29b4a4979360ddfe6e7e3c416de96 Mon Sep 17 00:00:00 2001
From: Andrew Meadows <andrew@highfidelity.io>
Date: Sun, 27 Mar 2016 16:33:53 -0700
Subject: [PATCH] check for time machine

---
 libraries/entities/src/ParticleEffectEntityItem.cpp |  6 ++++++
 libraries/script-engine/src/ScriptEngine.cpp        | 11 +++++++----
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/libraries/entities/src/ParticleEffectEntityItem.cpp b/libraries/entities/src/ParticleEffectEntityItem.cpp
index 4b798cbcd9..02a86ab952 100644
--- a/libraries/entities/src/ParticleEffectEntityItem.cpp
+++ b/libraries/entities/src/ParticleEffectEntityItem.cpp
@@ -561,6 +561,12 @@ bool ParticleEffectEntityItem::needsToCallUpdate() const {
 
 void ParticleEffectEntityItem::update(const quint64& now) {
 
+    // we check for 'now' in the past in case users set their clock backward
+    if (now < _lastSimulated) {
+        _lastSimulated = now;
+        return;
+    }
+
     float deltaTime = (float)(now - _lastSimulated) / (float)USECS_PER_SECOND;
     _lastSimulated = now;
 
diff --git a/libraries/script-engine/src/ScriptEngine.cpp b/libraries/script-engine/src/ScriptEngine.cpp
index 3f403b3677..a6541dc031 100644
--- a/libraries/script-engine/src/ScriptEngine.cpp
+++ b/libraries/script-engine/src/ScriptEngine.cpp
@@ -673,11 +673,14 @@ void ScriptEngine::run() {
         }
 
         qint64 now = usecTimestampNow();
-        float deltaTime = (float) (now - lastUpdate) / (float) USECS_PER_SECOND;
 
-        if (!_isFinished) {
-            if (_wantSignals) {
-                emit update(deltaTime);
+        // we check for 'now' in the past in case people set their clock back
+        if (lastUpdate < now) {
+            float deltaTime = (float) (now - lastUpdate) / (float) USECS_PER_SECOND;
+            if (!_isFinished) {
+                if (_wantSignals) {
+                    emit update(deltaTime);
+                }
             }
         }
         lastUpdate = now;