diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index b13729cb96f..a5dba7775f1 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1696,8 +1696,7 @@ GCRuntime::computeHeapGrowthFactor(size_t lastBytes)
         factor = lowFrequencyHeapGrowth;
     } else {
         JS_ASSERT(highFrequencyHighLimitBytes > highFrequencyLowLimitBytes);
-        uint64_t now = PRMJ_Now();
-        if (lastGCTime && lastGCTime + highFrequencyTimeThreshold * PRMJ_USEC_PER_MSEC > now) {
+        if (highFrequencyGC) {
             if (lastBytes <= highFrequencyLowLimitBytes) {
                 factor = highFrequencyHeapGrowthMax;
             } else if (lastBytes >= highFrequencyHighLimitBytes) {
@@ -1710,10 +1709,8 @@ GCRuntime::computeHeapGrowthFactor(size_t lastBytes)
                 JS_ASSERT(factor <= highFrequencyHeapGrowthMax
                           && factor >= highFrequencyHeapGrowthMin);
             }
-            highFrequencyGC = true;
         } else {
             factor = lowFrequencyHeapGrowth;
-            highFrequencyGC = false;
         }
     }
 
@@ -4476,6 +4473,10 @@ GCRuntime::endSweepPhase(JSGCInvocationKind gckind, bool lastGC)
             sweepZones(&fop, lastGC);
     }
 
+    uint64_t currentTime = PRMJ_Now();
+    highFrequencyGC = dynamicHeapGrowth && lastGCTime &&
+        lastGCTime + highFrequencyTimeThreshold * PRMJ_USEC_PER_MSEC > currentTime;
+
     for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
         zone->setGCLastBytes(zone->gcBytes, gckind);
         if (zone->isCollecting()) {
@@ -4509,7 +4510,7 @@ GCRuntime::endSweepPhase(JSGCInvocationKind gckind, bool lastGC)
 
     finishMarkingValidation();
 
-    lastGCTime = PRMJ_Now();
+    lastGCTime = currentTime;
 }
 
 /* Start a new heap session. */