diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 09ef784a04..edff454491 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -4122,6 +4122,7 @@ func (pp *p) destroy() {
 		// The world is stopped so we don't need to hold timersLock.
 		moveTimers(plocal, pp.timers)
 		pp.timers = nil
+		pp.adjustTimers = 0
 	}
 	// If there's a background worker, make it runnable and put
 	// it on the global queue so it can clean itself up.
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index f44cd2fb14..b57ae75baf 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -607,6 +607,12 @@ type p struct {
 	// Must hold timersLock to access.
 	timers []*timer
 
+	// Number of timerModifiedEarlier timers on P's heap.
+	// This should only be modified while holding timersLock,
+	// or while the timer status is in a transient state
+	// such as timerModifying.
+	adjustTimers uint32
+
 	pad cpu.CacheLinePad
 }
 
diff --git a/src/runtime/time.go b/src/runtime/time.go
index f2dd40e6b4..54cbb0b6e4 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -131,6 +131,16 @@ type timersBucket struct {
 //   timerRunning    -> wait until status changes
 //   timerMoving     -> wait until status changes
 //   timerModifying  -> panic: concurrent deltimer/modtimer calls
+// modtimer:
+//   timerWaiting    -> timerModifying -> timerModifiedXX
+//   timerModifiedXX -> timerModifying -> timerModifiedYY
+//   timerNoStatus   -> timerWaiting
+//   timerRemoved    -> timerWaiting
+//   timerRunning    -> wait until status changes
+//   timerMoving     -> wait until status changes
+//   timerRemoving   -> wait until status changes
+//   timerDeleted    -> panic: concurrent modtimer/deltimer calls
+//   timerModifying  -> panic: concurrent modtimer calls
 
 // Values for the timer status field.
 const (
@@ -270,6 +280,11 @@ func addtimer(t *timer) {
 	}
 	t.status = timerWaiting
 
+	addInitializedTimer(t)
+}
+
+// addInitializedTimer adds an initialized timer to the current P.
+func addInitializedTimer(t *timer) {
 	when := t.when
 
 	pp := getg().m.p.ptr()
@@ -363,7 +378,9 @@ func deltimer(t *timer) bool {
 				return true
 			}
 		case timerModifiedEarlier:
+			tpp := t.pp.ptr()
 			if atomic.Cas(&t.status, s, timerModifying) {
+				atomic.Xadd(&tpp.adjustTimers, -1)
 				if !atomic.Cas(&t.status, timerModifying, timerDeleted) {
 					badTimer()
 				}
@@ -438,12 +455,94 @@ func (tb *timersBucket) deltimerLocked(t *timer) (removed, ok bool) {
 	return true, ok
 }
 
+// modtimer modifies an existing timer.
+// This is called by the netpoll code.
 func modtimer(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) {
 	if oldTimers {
 		modtimerOld(t, when, period, f, arg, seq)
 		return
 	}
-	throw("new modtimer not yet implemented")
+
+	if when < 0 {
+		when = maxWhen
+	}
+
+	status := uint32(timerNoStatus)
+	wasRemoved := false
+loop:
+	for {
+		switch status = atomic.Load(&t.status); status {
+		case timerWaiting, timerModifiedEarlier, timerModifiedLater:
+			if atomic.Cas(&t.status, status, timerModifying) {
+				break loop
+			}
+		case timerNoStatus, timerRemoved:
+			// Timer was already run and t is no longer in a heap.
+			// Act like addtimer.
+			wasRemoved = true
+			atomic.Store(&t.status, timerWaiting)
+			break loop
+		case timerRunning, timerRemoving, timerMoving:
+			// The timer is being run or moved, by a different P.
+			// Wait for it to complete.
+			osyield()
+		case timerDeleted:
+			// Simultaneous calls to modtimer and deltimer.
+			badTimer()
+		case timerModifying:
+			// Multiple simultaneous calls to modtimer.
+			badTimer()
+		default:
+			badTimer()
+		}
+	}
+
+	t.period = period
+	t.f = f
+	t.arg = arg
+	t.seq = seq
+
+	if wasRemoved {
+		t.when = when
+		addInitializedTimer(t)
+	} else {
+		// The timer is in some other P's heap, so we can't change
+		// the when field. If we did, the other P's heap would
+		// be out of order. So we put the new when value in the
+		// nextwhen field, and let the other P set the when field
+		// when it is prepared to resort the heap.
+		t.nextwhen = when
+
+		newStatus := uint32(timerModifiedLater)
+		if when < t.when {
+			newStatus = timerModifiedEarlier
+		}
+
+		// Update the adjustTimers field.  Subtract one if we
+		// are removing a timerModifiedEarlier, add one if we
+		// are adding a timerModifiedEarlier.
+		tpp := t.pp.ptr()
+		adjust := int32(0)
+		if status == timerModifiedEarlier {
+			adjust--
+		}
+		if newStatus == timerModifiedEarlier {
+			adjust++
+		}
+		if adjust != 0 {
+			atomic.Xadd(&tpp.adjustTimers, adjust)
+		}
+
+		// Set the new status of the timer.
+		if !atomic.Cas(&t.status, timerModifying, newStatus) {
+			badTimer()
+		}
+
+		// If the new status is earlier, wake up the poller.
+		if newStatus == timerModifiedEarlier {
+			wakeNetPoller(when)
+		}
+	}
 }
 
 func modtimerOld(t *timer, when, period int64, f func(interface{}, uintptr), arg interface{}, seq uintptr) {