diff --git a/homeassistant/__init__.py b/homeassistant/__init__.py
index a2be3780619..8bb4a0b8b1f 100644
--- a/homeassistant/__init__.py
+++ b/homeassistant/__init__.py
@@ -418,17 +418,13 @@ class State(object):
         self.entity_id = entity_id
         self.state = state
         self.attributes = attributes or {}
-        last_changed = last_changed or dt.datetime.now()
 
         # Strip microsecond from last_changed else we cannot guarantee
         # state == State.from_dict(state.as_dict())
         # This behavior occurs because to_dict uses datetime_to_str
-        # which strips microseconds
-        if last_changed.microsecond:
-            self.last_changed = last_changed - dt.timedelta(
-                microseconds=last_changed.microsecond)
-        else:
-            self.last_changed = last_changed
+        # which does not preserve microseconds
+        self.last_changed = util.strip_microseconds(
+            last_changed or dt.datetime.now())
 
     def copy(self):
         """ Creates a copy of itself. """
@@ -504,6 +500,19 @@ class StateMachine(object):
         # Make a copy so people won't mutate the state
         return state.copy() if state else None
 
+    def get_since(self, point_in_time):
+        """
+        Returns all states that have been changed since point_in_time.
+
+        Note: States keep track of last_changed -without- microseconds.
+        Therefore your point_in_time will also be stripped of microseconds.
+        """
+        point_in_time = util.strip_microseconds(point_in_time)
+
+        with self._lock:
+            return [state for state in self._states.values()
+                    if state.last_changed >= point_in_time]
+
     def is_state(self, entity_id, state):
         """ Returns True if entity exists and is specified state. """
         return (entity_id in self._states and
diff --git a/homeassistant/const.py b/homeassistant/const.py
index b8183edc2d0..49f39b11c94 100644
--- a/homeassistant/const.py
+++ b/homeassistant/const.py
@@ -39,7 +39,6 @@ ATTR_SERVICE = "service"
 
 # Data for a SERVICE_EXECUTED event
 ATTR_SERVICE_CALL_ID = "service_call_id"
-ATTR_RESULT = "result"
 
 # Contains one string or a list of strings, each being an entity id
 ATTR_ENTITY_ID = 'entity_id'
diff --git a/homeassistant/helpers.py b/homeassistant/helpers.py
index f673cc5345f..cdf645cb75f 100644
--- a/homeassistant/helpers.py
+++ b/homeassistant/helpers.py
@@ -1,6 +1,8 @@
 """
 Helper methods for components within Home Assistant.
 """
+from datetime import datetime
+
 from homeassistant import NoEntitySpecifiedError
 
 from homeassistant.loader import get_component
@@ -33,6 +35,25 @@ def extract_entity_ids(hass, service):
     return entity_ids
 
 
+# pylint: disable=too-few-public-methods, attribute-defined-outside-init
+class TrackStates(object):
+    """
+    Records the time when the with-block is entered. Will add all states
+    that have changed since the start time to the return list when with-block
+    is exited.
+    """
+    def __init__(self, hass):
+        self.hass = hass
+        self.states = []
+
+    def __enter__(self):
+        self.now = datetime.now()
+        return self.states
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        self.states.extend(self.hass.states.get_since(self.now))
+
+
 def validate_config(config, items, logger):
     """
     Validates if all items are available in the configuration.
diff --git a/homeassistant/util.py b/homeassistant/util.py
index 4598a230c65..fbf686c39ab 100644
--- a/homeassistant/util.py
+++ b/homeassistant/util.py
@@ -8,7 +8,7 @@ import collections
 from itertools import chain
 import threading
 import queue
-from datetime import datetime
+from datetime import datetime, timedelta
 import re
 import enum
 import socket
@@ -57,6 +57,14 @@ def str_to_datetime(dt_str):
         return None
 
 
+def strip_microseconds(dattim):
+    """ Returns a copy of dattime object but with microsecond set to 0. """
+    if dattim.microsecond:
+        return dattim - timedelta(microseconds=dattim.microsecond)
+    else:
+        return dattim
+
+
 def split_entity_id(entity_id):
     """ Splits a state entity_id into domain, object_id. """
     return entity_id.split(".", 1)