diff --git a/homeassistant/components/media_player/sonos.py b/homeassistant/components/media_player/sonos.py
index 2e3f9b5d38e..fcb650ce145 100644
--- a/homeassistant/components/media_player/sonos.py
+++ b/homeassistant/components/media_player/sonos.py
@@ -22,14 +22,13 @@ from homeassistant.components.media_player import (
     SUPPORT_PLAY)
 from homeassistant.const import (
     STATE_IDLE, STATE_PAUSED, STATE_PLAYING, STATE_OFF, ATTR_ENTITY_ID,
-    CONF_HOSTS)
+    CONF_HOSTS, ATTR_TIME)
 from homeassistant.config import load_yaml_config_file
 import homeassistant.helpers.config_validation as cv
 from homeassistant.util.dt import utcnow
 
 REQUIREMENTS = ['SoCo==0.12']
 
-
 _LOGGER = logging.getLogger(__name__)
 
 # The soco library is excessively chatty when it comes to logging and
@@ -68,7 +67,6 @@ ATTR_ALARM_ID = 'alarm_id'
 ATTR_VOLUME = 'volume'
 ATTR_ENABLED = 'enabled'
 ATTR_INCLUDE_LINKED_ZONES = 'include_linked_zones'
-ATTR_TIME = 'time'
 ATTR_MASTER = 'master'
 ATTR_WITH_GROUP = 'with_group'
 
diff --git a/homeassistant/components/sensor/imap_email_content.py b/homeassistant/components/sensor/imap_email_content.py
index 99613f0f7e8..64f10ad64a7 100644
--- a/homeassistant/components/sensor/imap_email_content.py
+++ b/homeassistant/components/sensor/imap_email_content.py
@@ -15,7 +15,7 @@ from homeassistant.helpers.entity import Entity
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.const import (
     CONF_NAME, CONF_PORT, CONF_USERNAME, CONF_PASSWORD, CONF_VALUE_TEMPLATE,
-    CONTENT_TYPE_TEXT_PLAIN)
+    CONTENT_TYPE_TEXT_PLAIN, ATTR_DATE)
 import homeassistant.helpers.config_validation as cv
 
 _LOGGER = logging.getLogger(__name__)
@@ -25,7 +25,6 @@ CONF_SENDERS = 'senders'
 
 ATTR_FROM = 'from'
 ATTR_BODY = 'body'
-ATTR_DATE = 'date'
 ATTR_SUBJECT = 'subject'
 
 DEFAULT_PORT = 993
@@ -59,7 +58,7 @@ def setup_platform(hass, config, add_devices, discovery_info=None):
         return False
 
 
-class EmailReader:
+class EmailReader(object):
     """A class to read emails from an IMAP server."""
 
     def __init__(self, user, password, server, port):
diff --git a/homeassistant/components/sensor/pvoutput.py b/homeassistant/components/sensor/pvoutput.py
index 251c117f191..cb3d2d1427a 100644
--- a/homeassistant/components/sensor/pvoutput.py
+++ b/homeassistant/components/sensor/pvoutput.py
@@ -6,6 +6,7 @@ https://home-assistant.io/components/sensor.pvoutput/
 """
 import logging
 from collections import namedtuple
+from datetime import timedelta
 
 import voluptuous as vol
 
@@ -14,13 +15,12 @@ from homeassistant.helpers.entity import Entity
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.components.sensor.rest import RestData
 from homeassistant.const import (
-    ATTR_TEMPERATURE, CONF_API_KEY, CONF_NAME, STATE_UNKNOWN)
+    ATTR_TEMPERATURE, CONF_API_KEY, CONF_NAME, STATE_UNKNOWN, ATTR_DATE,
+    ATTR_TIME)
 
 _LOGGER = logging.getLogger(__name__)
 _ENDPOINT = 'http://pvoutput.org/service/r2/getstatus.jsp'
 
-ATTR_DATE = 'date'
-ATTR_TIME = 'time'
 ATTR_ENERGY_GENERATION = 'energy_generation'
 ATTR_POWER_GENERATION = 'power_generation'
 ATTR_ENERGY_CONSUMPTION = 'energy_consumption'
@@ -33,6 +33,8 @@ CONF_SYSTEM_ID = 'system_id'
 DEFAULT_NAME = 'PVOutput'
 DEFAULT_VERIFY_SSL = True
 
+SCAN_INTERVAL = timedelta(minutes=2)
+
 PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Required(CONF_API_KEY): cv.string,
     vol.Required(CONF_SYSTEM_ID): cv.string,
@@ -60,7 +62,7 @@ def setup_platform(hass, config, add_devices, discovery_info=None):
         _LOGGER.error("Unable to fetch data from PVOutput")
         return False
 
-    add_devices([PvoutputSensor(rest, name)])
+    add_devices([PvoutputSensor(rest, name)], True)
 
 
 # pylint: disable=no-member
@@ -71,15 +73,13 @@ class PvoutputSensor(Entity):
         """Initialize a PVOutput sensor."""
         self.rest = rest
         self._name = name
-        self.pvcoutput = False
+        self.pvcoutput = None
         self.status = namedtuple(
             'status', [ATTR_DATE, ATTR_TIME, ATTR_ENERGY_GENERATION,
                        ATTR_POWER_GENERATION, ATTR_ENERGY_CONSUMPTION,
                        ATTR_POWER_CONSUMPTION, ATTR_EFFICIENCY,
                        ATTR_TEMPERATURE, ATTR_VOLTAGE])
 
-        self.update()
-
     @property
     def name(self):
         """Return the name of the sensor."""
diff --git a/homeassistant/components/sensor/waqi.py b/homeassistant/components/sensor/waqi.py
index 11e1f17c2ba..3b8e541143d 100644
--- a/homeassistant/components/sensor/waqi.py
+++ b/homeassistant/components/sensor/waqi.py
@@ -10,7 +10,7 @@ from datetime import timedelta
 import voluptuous as vol
 
 from homeassistant.const import (
-    ATTR_ATTRIBUTION, ATTR_TEMPERATURE, STATE_UNKNOWN)
+    ATTR_ATTRIBUTION, ATTR_TIME, ATTR_TEMPERATURE, STATE_UNKNOWN)
 from homeassistant.helpers import config_validation as cv
 from homeassistant.helpers.config_validation import PLATFORM_SCHEMA
 from homeassistant.helpers.entity import Entity
@@ -28,7 +28,6 @@ ATTR_PM10 = 'pm_10'
 ATTR_PM2_5 = 'pm_2_5'
 ATTR_PRESSURE = 'pressure'
 ATTR_SULFUR_DIOXIDE = 'sulfur_dioxide'
-ATTR_TIME = 'time'
 ATTRIBUTION = 'Data provided by the World Air Quality Index project'
 
 CONF_LOCATIONS = 'locations'
diff --git a/homeassistant/const.py b/homeassistant/const.py
index bef4a9387f5..baafb57e934 100644
--- a/homeassistant/const.py
+++ b/homeassistant/const.py
@@ -202,8 +202,10 @@ STATE_UNAVAILABLE = 'unavailable'
 # Attribution
 ATTR_ATTRIBUTION = 'attribution'
 
-# Contains current time for a TIME_CHANGED event
+# Contains time-related attributes
 ATTR_NOW = 'now'
+ATTR_DATE = 'date'
+ATTR_TIME = 'time'
 
 # Contains domain, service for a SERVICE_CALL event
 ATTR_DOMAIN = 'domain'