diff --git a/lib/logstash/filters/elapsed.rb b/lib/logstash/filters/elapsed.rb
new file mode 100644
index 000000000..64881f5b9
--- /dev/null
+++ b/lib/logstash/filters/elapsed.rb
@@ -0,0 +1,256 @@
+# elapsed filter
+#
+# This filter tracks a pair of start/end events and calculates the elapsed
+# time between them.
+
+require "logstash/filters/base"
+require "logstash/namespace"
+require 'thread'
+
+# The elapsed filter tracks a pair of start/end events and uses their
+# timestamps to calculate the elapsed time between them.
+#
+# The filter has been developed to track the execution time of processes and
+# other long tasks.
+#
+# The configuration looks like this:
+#
+#     filter {
+#       elapsed {
+#         start_tag => "start event tag"
+#         end_tag => "end event tag"
+#         unique_id_field => "id field name"
+#         timeout => seconds
+#         new_event_on_match => true/false
+#       }
+#     }
+#
+# The events managed by this filter must have some particular properties.
+# The event describing the start of the task (the "start event") must contain
+# a tag equal to 'start_tag'. On the other side, the event describing the end
+# of the task (the "end event") must contain a tag equal to 'end_tag'. Both
+# these two kinds of event need to own an ID field which identify uniquely that
+# particular task. The name of this field is stored in 'unique_id_field'.
+#
+# You can use a Grok filter to prepare the events for the elapsed filter.
+# An example of configuration can be:
+#
+#     filter {
+#       grok {
+#         match => ["message", "%{TIMESTAMP_ISO8601} START id: (?<task_id>.*)"]
+#         add_tag => [ "taskStarted" ]
+#       }
+#
+#       grok {
+#         match => ["message", "%{TIMESTAMP_ISO8601} END id: (?<task_id>.*)"]
+#         add_tag => [ "taskTerminated"]
+#       }
+#
+#       elapsed {
+#         start_tag => "taskStarted"
+#         end_tag => "taskTerminated"
+#         unique_id_field => "task_id"
+#       }
+#     }
+#
+# The elapsed filter collects all the "start events". If two, or more, "start
+# events" have the same ID, only the first one is recorded, the others are
+# discarded.
+#
+# When an "end event" matching a previously collected "start event" is
+# received, there is a match. The configuration property 'new_event_on_match'
+# tells where to insert the elapsed information: they can be added to the
+# "end event" or a new "match event" can be created. Both events store the
+# following information:
+# - the tags "elapsed" and "elapsed.match"
+# - the field "elapsed.time" with the difference, in seconds, between
+#   the two events timestamps
+# - an ID filed with the task ID
+# - the field "elapsed.timestamp_start" with the timestamp of the "start event"
+#
+# If the "end event" does not arrive before "timeout" seconds, the
+# "start event" is discarded and an "expired event" is generated. This event
+# contains:
+# - the tags "elapsed" and "elapsed.expired_error"
+# - a field called "elapsed.time" with the age, in seconds, of the
+#   "start event"
+# - an ID filed with the task ID
+# - the field "elapsed.timestamp_start" with the timestamp of the "start event"
+#
+class LogStash::Filters::Elapsed < LogStash::Filters::Base
+  PREFIX = "elapsed."
+  ELAPSED_FIELD = PREFIX + "time"
+  TIMESTAMP_START_EVENT_FIELD = PREFIX + "timestamp_start"
+  HOST_FIELD = "host"
+
+  ELAPSED_TAG = "elapsed"
+  EXPIRED_ERROR_TAG = PREFIX + "expired_error"
+  END_WITHOUT_START_TAG = PREFIX + "end_wtihout_start"
+  MATCH_TAG = PREFIX + "match"
+
+  config_name "elapsed"
+  milestone 1
+
+  # The name of the tag identifying the "start event"
+  config :start_tag, :validate => :string, :required => true
+
+  # The name of the tag identifying the "end event"
+  config :end_tag, :validate => :string, :required => true
+
+  # The name of the field containing the task ID.
+  # This value must uniquely identify the task in the system, otherwise
+  # it's impossible to match the couple of events.
+  config :unique_id_field, :validate => :string, :required => true
+
+  # The amount of seconds after an "end event" can be considered lost.
+  # The corresponding "start event" is discarded and an "expired event"
+  # is generated. The default value is 30 minutes (1800 seconds).
+  config :timeout, :validate => :number, :required => false, :default => 1800
+
+  # This property manage what to do when an "end event" matches a "start event".
+  # If it's set to 'false' (default value), the elapsed information are added
+  # to the "end event"; if it's set to 'true' a new "match event" is created.
+  config :new_event_on_match, :validate => :boolean, :required => false, :default => false
+
+  public
+  def register
+    @mutex = Mutex.new
+    # This is the state of the filter. The keys are the "unique_id_field",
+    # the values are couples of values: <start event, age>
+    @start_events = {}
+
+    @logger.info("Elapsed, timeout: #{@timeout} seconds")
+  end
+
+  # Getter method used for the tests
+  def start_events
+    @start_events
+  end
+
+  def filter(event)
+    return unless filter?(event)
+
+    unique_id = event[@unique_id_field]
+    return if unique_id.nil?
+
+    if(start_event?(event))
+      filter_matched(event)
+      @logger.info("Elapsed, 'start event' received", start_tag: @start_tag, unique_id_field: @unique_id_field)
+
+      @mutex.synchronize do
+        unless(@start_events.has_key?(unique_id))
+          @start_events[unique_id] = LogStash::Filters::Elapsed::Element.new(event)
+        end
+      end
+
+    elsif(end_event?(event))
+      filter_matched(event)
+      @logger.info("Elapsed, 'end event' received", end_tag: @end_tag, unique_id_field: @unique_id_field)
+
+      @mutex.lock
+      if(@start_events.has_key?(unique_id))
+        start_event = @start_events.delete(unique_id).event
+        @mutex.unlock
+        elapsed = event["@timestamp"] - start_event["@timestamp"]
+        if(@new_event_on_match)
+          elapsed_event = new_elapsed_event(elapsed, unique_id, start_event["@timestamp"])
+          filter_matched(elapsed_event)
+          yield elapsed_event if block_given?
+        else
+          return add_elapsed_info(event, elapsed, unique_id, start_event["@timestamp"])
+        end
+      else
+        @mutex.unlock
+        # The "start event" did not arrive.
+        event.tag(END_WITHOUT_START_TAG)
+      end
+    end
+  end # def filter
+
+  # The method is invoked by LogStash every 5 seconds.
+  def flush()
+    expired_elements = []
+
+    @mutex.synchronize do
+      increment_age_by(5)
+      expired_elements = remove_expired_elements()
+    end
+
+    return create_expired_events_from(expired_elements)
+  end
+
+  private
+  def increment_age_by(seconds)
+    @start_events.each_pair do |key, element|
+      element.age += seconds
+    end
+  end
+
+  # Remove the expired "start events" from the internal
+  # buffer and return them.
+  def remove_expired_elements()
+    expired = []
+    @start_events.delete_if do |key, element|
+      if(element.age >= @timeout)
+        expired << element
+        next true
+      end
+      next false
+    end
+
+    return expired
+  end
+
+  def create_expired_events_from(expired_elements)
+    events = []
+    expired_elements.each do |element|
+      error_event = LogStash::Event.new
+      error_event.tag(ELAPSED_TAG)
+      error_event.tag(EXPIRED_ERROR_TAG)
+
+      error_event[HOST_FIELD] = Socket.gethostname
+      error_event[@unique_id_field] = element.event[@unique_id_field]
+      error_event[ELAPSED_FIELD] = element.age
+      error_event[TIMESTAMP_START_EVENT_FIELD] = element.event["@timestamp"]
+
+      events << error_event
+      filter_matched(error_event)
+    end
+
+    return events
+  end
+
+  def start_event?(event)
+    return (event["tags"] != nil && event["tags"].include?(@start_tag))
+  end
+
+  def end_event?(event)
+    return (event["tags"] != nil && event["tags"].include?(@end_tag))
+  end
+
+  def new_elapsed_event(elapsed_time, unique_id, timestamp_start_event)
+      new_event = LogStash::Event.new
+      new_event[HOST_FIELD] = Socket.gethostname
+      return add_elapsed_info(new_event, elapsed_time, unique_id, timestamp_start_event)
+  end
+
+  def add_elapsed_info(event, elapsed_time, unique_id, timestamp_start_event)
+      event.tag(ELAPSED_TAG)
+      event.tag(MATCH_TAG)
+
+      event[ELAPSED_FIELD] = elapsed_time
+      event[@unique_id_field] = unique_id
+      event[TIMESTAMP_START_EVENT_FIELD] = timestamp_start_event
+
+      return event
+  end
+end # class LogStash::Filters::Elapsed
+
+class LogStash::Filters::Elapsed::Element
+  attr_accessor :event, :age
+
+  def initialize(event)
+    @event = event
+    @age = 0
+  end
+end
diff --git a/spec/filters/elapsed.rb b/spec/filters/elapsed.rb
new file mode 100644
index 000000000..067705a4b
--- /dev/null
+++ b/spec/filters/elapsed.rb
@@ -0,0 +1,294 @@
+require 'socket'
+require "logstash/filters/elapsed"
+
+describe LogStash::Filters::Elapsed do
+  START_TAG = "startTag"
+  END_TAG   = "endTag"
+  ID_FIELD  = "uniqueIdField"
+
+  def event(data)
+    data["message"] ||= "Log message"
+    LogStash::Event.new(data)
+  end
+
+  def start_event(data)
+    data["tags"] ||= []
+    data["tags"] << START_TAG
+    event(data)
+  end
+
+  def end_event(data = {})
+    data["tags"] ||= []
+    data["tags"] << END_TAG
+    event(data)
+  end
+
+  before(:each) do
+    setup_filter()
+  end
+
+  def setup_filter(config = {})
+    @config = {"start_tag" => START_TAG, "end_tag" => END_TAG, "unique_id_field" => ID_FIELD}
+    @config.merge!(config)
+    @filter = LogStash::Filters::Elapsed.new(@config)
+    @filter.register
+  end
+
+  context "General validation" do
+    describe "receiving an event without start or end tag" do
+      it "does not record it" do
+        @filter.filter(event("message" => "Log message"))
+        insist { @filter.start_events.size } == 0
+      end
+    end
+
+    describe "receiving an event with a different start/end tag from the ones specified in the configuration" do
+      it "does not record it" do
+        @filter.filter(event("tags" => ["tag1", "tag2"]))
+        insist { @filter.start_events.size } == 0
+      end
+    end
+  end
+
+  context "Start event" do
+    describe "receiving an event with a valid start tag" do
+      describe "but without an unique id field" do
+        it "does not record it" do
+          @filter.filter(event("tags" => ["tag1", START_TAG]))
+          insist { @filter.start_events.size } == 0
+        end
+      end
+
+      describe "and a valid id field" do
+        it "records it" do
+          event = start_event(ID_FIELD => "id123")
+          @filter.filter(event)
+
+          insist { @filter.start_events.size } == 1
+          insist { @filter.start_events["id123"].event } == event
+        end
+      end
+    end
+
+    describe "receiving two 'start events' for the same id field" do
+      it "keeps the first one and does not save the second one" do
+          args = {"tags" => [START_TAG], ID_FIELD => "id123"}
+          first_event = event(args)
+          second_event = event(args)
+
+          @filter.filter(first_event)
+          @filter.filter(second_event)
+
+          insist { @filter.start_events.size } == 1
+          insist { @filter.start_events["id123"].event } == first_event
+      end
+    end
+  end
+
+  context "End event" do
+    describe "receiving an event with a valid end tag" do
+      describe "and without an id" do
+        it "does nothing" do
+          insist { @filter.start_events.size } == 0
+          @filter.filter(end_event())
+          insist { @filter.start_events.size } == 0
+        end
+      end
+
+      describe "and with an id" do
+        describe "but without a previous 'start event'" do
+          it "adds a tag 'elapsed.end_witout_start' to the 'end event'" do
+            end_event = end_event(ID_FIELD => "id_123")
+
+            @filter.filter(end_event)
+
+            insist { end_event["tags"].include?("elapsed.end_wtihout_start") } == true
+          end
+        end
+      end
+    end
+  end
+
+  context "Start/end events interaction" do
+    describe "receiving a 'start event'" do
+      before(:each) do
+        @id_value = "id_123"
+        @start_event = start_event(ID_FIELD => @id_value)
+        @filter.filter(@start_event)
+      end
+
+      describe "and receiving an event with a valid end tag" do
+        describe "and without an id" do
+          it "does nothing" do
+            @filter.filter(end_event())
+            insist { @filter.start_events.size } == 1
+            insist { @filter.start_events[@id_value].event } == @start_event
+          end
+        end
+
+        describe "and an id different from the one of the 'start event'" do
+          it "does nothing" do
+            different_id_value = @id_value + "_different"
+            @filter.filter(end_event(ID_FIELD => different_id_value))
+
+            insist { @filter.start_events.size } == 1
+            insist { @filter.start_events[@id_value].event } == @start_event
+          end
+        end
+
+        describe "and the same id of the 'start event'" do
+          it "deletes the recorded 'start event'" do
+            insist { @filter.start_events.size } == 1
+
+            @filter.filter(end_event(ID_FIELD => @id_value))
+
+            insist { @filter.start_events.size } == 0
+          end
+
+          shared_examples_for "match event" do
+            it "contains the tag 'elapsed'" do
+              insist { @match_event["tags"].include?("elapsed") } == true
+            end
+
+            it "contains the tag tag 'elapsed.match'" do
+              insist { @match_event["tags"].include?("elapsed.match") } == true
+            end
+
+            it "contains an 'elapsed.time field' with the elapsed time" do
+              insist { @match_event["elapsed.time"] } == 10
+            end
+
+            it "contains an 'elapsed.timestamp_start field' with the timestamp of the 'start event'" do
+              insist { @match_event["elapsed.timestamp_start"] } == @start_event["@timestamp"]
+            end
+
+            it "contains an 'id field'" do
+              insist { @match_event[ID_FIELD] } == @id_value
+            end
+          end
+
+          context "if 'new_event_on_match' is set to 'true'" do
+            before(:each) do
+              # I need to create a new filter because I need to set
+              # the config property 'new_event_on_match" to 'true'.
+              setup_filter("new_event_on_match" => true)
+              @start_event = start_event(ID_FIELD => @id_value)
+              @filter.filter(@start_event)
+
+              end_timestamp = @start_event["@timestamp"] + 10
+              end_event = end_event(ID_FIELD => @id_value, "@timestamp" => end_timestamp)
+              @filter.filter(end_event) do |new_event|
+                @match_event = new_event
+              end
+            end
+
+            context "creates a new event that" do
+              it_behaves_like "match event"
+
+              it "contains the 'host field'" do
+                insist { @match_event["host"] } == Socket.gethostname
+              end
+            end
+          end
+
+          context "if 'new_event_on_match' is set to 'false'" do
+            before(:each) do
+              end_timestamp = @start_event["@timestamp"] + 10
+              end_event = end_event(ID_FIELD => @id_value, "@timestamp" => end_timestamp)
+              @filter.filter(end_event)
+
+              @match_event = end_event
+            end
+
+            context "modifies the 'end event' that" do
+              it_behaves_like "match event"
+            end
+          end
+
+        end
+      end
+    end
+  end
+
+  describe "#flush" do
+    def setup(timeout = 1000)
+      @config["timeout"] = timeout
+      @filter = LogStash::Filters::Elapsed.new(@config)
+      @filter.register
+
+      @start_event_1 = start_event(ID_FIELD => "1")
+      @start_event_2 = start_event(ID_FIELD => "2")
+      @start_event_3 = start_event(ID_FIELD => "3")
+
+      @filter.filter(@start_event_1)
+      @filter.filter(@start_event_2)
+      @filter.filter(@start_event_3)
+
+      # Force recorded events to different ages
+      @filter.start_events["2"].age = 25
+      @filter.start_events["3"].age = 26
+    end
+
+    it "increments the 'age' of all the recorded 'start events' by 5 seconds" do
+      setup()
+      old_age = ages()
+
+      @filter.flush()
+
+      ages().each_with_index do |new_age, i|
+        insist { new_age } == (old_age[i] + 5)
+      end
+    end
+
+    def ages()
+      @filter.start_events.each_value.map{|element| element.age }
+    end
+
+    context "if the 'timeout interval' is set to 30 seconds" do
+      before(:each) do
+        setup(30)
+
+        @expired_events = @filter.flush()
+
+        insist { @filter.start_events.size } == 1
+        insist { @expired_events.size } == 2
+      end
+
+      it "deletes the recorded 'start events' with 'age' greater, or equal to, the timeout" do
+        insist { @filter.start_events.key?("1") } == true
+        insist { @filter.start_events.key?("2") } == false
+        insist { @filter.start_events.key?("3") } == false
+      end
+
+      it "creates a new event with tag 'elapsed.expired_error' for each expired 'start event'" do
+        insist { @expired_events[0]["tags"].include?("elapsed.expired_error") } == true
+        insist { @expired_events[1]["tags"].include?("elapsed.expired_error") } == true
+      end
+
+      it "creates a new event with tag 'elapsed' for each expired 'start event'" do
+        insist { @expired_events[0]["tags"].include?("elapsed") } == true
+        insist { @expired_events[1]["tags"].include?("elapsed") } == true
+      end
+
+      it "creates a new event containing the 'id field' of the expired 'start event'" do
+        insist { @expired_events[0][ID_FIELD] } == "2"
+        insist { @expired_events[1][ID_FIELD] } == "3"
+      end
+
+      it "creates a new event containing an 'elapsed.time field' with the age of the expired 'start event'" do
+        insist { @expired_events[0]["elapsed.time"] } == 30
+        insist { @expired_events[1]["elapsed.time"] } == 31
+      end
+
+      it "creates a new event containing an 'elapsed.timestamp_start field' with the timestamp of the expired 'start event'" do
+        insist { @expired_events[0]["elapsed.timestamp_start"] } == @start_event_2["@timestamp"]
+        insist { @expired_events[1]["elapsed.timestamp_start"] } == @start_event_3["@timestamp"]
+      end
+
+      it "creates a new event containing a 'host field' for each expired 'start event'" do
+        insist { @expired_events[0]["host"] } == Socket.gethostname
+        insist { @expired_events[1]["host"] } == Socket.gethostname
+      end
+    end
+  end
+end