diff --git a/lib/concurrent/executor/timer_set.rb b/lib/concurrent/executor/timer_set.rb
index e93e703b5..d59d7cbe4 100644
--- a/lib/concurrent/executor/timer_set.rb
+++ b/lib/concurrent/executor/timer_set.rb
@@ -55,10 +55,10 @@ def post(intended_time, *args, &task)
           @queue.push(Task.new(time, args, task))
           @timer_executor.post(&method(:process_tasks))
         end
-
-        true
       end
 
+      @condition.signal
+      true
     end
 
     # For a timer, #kill is like an orderly shutdown, except we need to manually
@@ -129,8 +129,20 @@ def process_tasks
         interval = task.time - Time.now.to_f
 
         if interval <= 0
+          # We need to remove the task from the queue before passing
+          # it to the executor, to avoid race conditions where we pass
+          # the peek'ed task to the executor and then pop a different
+          # one that's been added in the meantime.
+          #
+          # Note that there's no race condition between the peek and
+          # this pop - this pop could retrieve a different task from
+          # the peek, but that task would be due to fire now anyway
+          # (because @queue is a priority queue, and this thread is
+          # the only reader, so whatever timer is at the head of the
+          # queue now must have the same pop time, or a closer one, as
+          # when we peeked).
+          task = mutex.synchronize { @queue.pop }
           @task_executor.post(*task.args, &task.op)
-          mutex.synchronize { @queue.pop }
         else
           mutex.synchronize do
             @condition.wait(mutex, [interval, 60].min)
diff --git a/spec/concurrent/executor/timer_set_spec.rb b/spec/concurrent/executor/timer_set_spec.rb
index 3458f093e..cf6adbd9b 100644
--- a/spec/concurrent/executor/timer_set_spec.rb
+++ b/spec/concurrent/executor/timer_set_spec.rb
@@ -35,6 +35,18 @@ module Concurrent
       expect(latch.wait(0.2)).to be_truthy
     end
 
+    it 'returns true when posting a task' do
+      expect(subject.post(0.1) { nil }).to be true
+    end
+
+    it 'executes a given task when given an interval in seconds, even if longer tasks have been scheduled' do
+      latch = CountDownLatch.new(1)
+      subject.post(0.5){ nil }
+      sleep 0.1
+      subject.post(0.1){ latch.count_down }
+      expect(latch.wait(0.2)).to be_truthy
+    end
+
     it 'passes all arguments to the task on execution' do
       expected = nil
       latch = CountDownLatch.new(1)