--- a/hotspot/src/share/vm/gc/g1/g1Analytics.cpp	Tue Aug 02 16:24:04 2016 -0400
+++ b/hotspot/src/share/vm/gc/g1/g1Analytics.cpp	Wed Aug 03 02:10:23 2016 +0000
@@ -316,8 +316,12 @@
   return get_new_size_prediction(_pending_cards_seq);
 }
 
+double G1Analytics::oldest_known_gc_end_time_sec() const {
+  return _recent_prev_end_times_for_all_gcs_sec->oldest();
+}
+
 double G1Analytics::last_known_gc_end_time_sec() const {
-  return _recent_prev_end_times_for_all_gcs_sec->oldest();
+  return _recent_prev_end_times_for_all_gcs_sec->last();
 }
 
 void G1Analytics::update_recent_gc_times(double end_time_sec,

--- a/hotspot/src/share/vm/gc/g1/g1Analytics.hpp	Tue Aug 02 16:24:04 2016 -0400
+++ b/hotspot/src/share/vm/gc/g1/g1Analytics.hpp	Wed Aug 03 02:10:23 2016 +0000
@@ -155,6 +155,7 @@
   void update_recent_gc_times(double end_time_sec, double elapsed_ms);
   void compute_pause_time_ratio(double interval_ms, double pause_time_ms);
 
+  double oldest_known_gc_end_time_sec() const;
   double last_known_gc_end_time_sec() const;
 };
 

--- a/hotspot/src/share/vm/gc/g1/g1CollectedHeap.cpp	Tue Aug 02 16:24:04 2016 -0400
+++ b/hotspot/src/share/vm/gc/g1/g1CollectedHeap.cpp	Wed Aug 03 02:10:23 2016 +0000
@@ -28,6 +28,7 @@
 #include "classfile/symbolTable.hpp"
 #include "code/codeCache.hpp"
 #include "code/icBuffer.hpp"
+#include "gc/g1/g1Analytics.hpp"
 #include "gc/g1/bufferingOopClosure.hpp"
 #include "gc/g1/concurrentG1Refine.hpp"
 #include "gc/g1/concurrentG1RefineThread.hpp"
@@ -2473,8 +2474,19 @@
 }
 
 jlong G1CollectedHeap::millis_since_last_gc() {
-  // assert(false, "NYI");
-  return 0;
+  jlong now = os::elapsed_counter() / NANOSECS_PER_MILLISEC;
+  const G1Analytics* analytics = _g1_policy->analytics();
+  double last = analytics->last_known_gc_end_time_sec();
+  jlong ret_val = now - (last * 1000);
+  if (ret_val < 0) {
+    // See the notes in GenCollectedHeap::millis_since_last_gc()
+    // for more information about the implementation.
+    log_warning(gc)("Detected clock going backwards. "
+      "Milliseconds since last GC would be " JLONG_FORMAT
+      ". returning zero instead.", ret_val);
+    return 0;
+  }
+  return ret_val;
 }
 
 void G1CollectedHeap::prepare_for_verify() {

--- a/hotspot/src/share/vm/gc/g1/g1DefaultPolicy.cpp	Tue Aug 02 16:24:04 2016 -0400
+++ b/hotspot/src/share/vm/gc/g1/g1DefaultPolicy.cpp	Wed Aug 03 02:10:23 2016 +0000
@@ -604,7 +604,7 @@
     _analytics->report_alloc_rate_ms(alloc_rate_ms);
 
     double interval_ms =
-      (end_time_sec - _analytics->last_known_gc_end_time_sec()) * 1000.0;
+      (end_time_sec - _analytics->oldest_known_gc_end_time_sec()) * 1000.0;
     _analytics->update_recent_gc_times(end_time_sec, pause_time_ms);
     _analytics->compute_pause_time_ratio(interval_ms, pause_time_ms);
   }

--- a/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp	Tue Aug 02 16:24:04 2016 -0400
+++ b/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp	Wed Aug 03 02:10:23 2016 +0000
@@ -1256,21 +1256,21 @@
 };
 
 jlong GenCollectedHeap::millis_since_last_gc() {
-  // We need a monotonically non-decreasing time in ms but
-  // os::javaTimeMillis() does not guarantee monotonicity.
+  // javaTimeNanos() is guaranteed to be monotonically non-decreasing
+  // provided the underlying platform provides such a time source
+  // (and it is bug free). So we still have to guard against getting
+  // back a time later than 'now'.
   jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
   GenTimeOfLastGCClosure tolgc_cl(now);
   // iterate over generations getting the oldest
   // time that a generation was collected
   generation_iterate(&tolgc_cl, false);
 
-  // javaTimeNanos() is guaranteed to be monotonically non-decreasing
-  // provided the underlying platform provides such a time source
-  // (and it is bug free). So we still have to guard against getting
-  // back a time later than 'now'.
   jlong retVal = now - tolgc_cl.time();
   if (retVal < 0) {
-    NOT_PRODUCT(log_warning(gc)("time warp: " JLONG_FORMAT, retVal);)
+    log_warning(gc)("Detected clock going backwards. "
+      "Milliseconds since last GC would be " JLONG_FORMAT
+      ". returning zero instead.", retVal);
     return 0;
   }
   return retVal;