diff --git a/homeassistant/components/sensor/cert_expiry.py b/homeassistant/components/sensor/cert_expiry.py
index dfc15510d6f..1ccaf2f6925 100644
--- a/homeassistant/components/sensor/cert_expiry.py
+++ b/homeassistant/components/sensor/cert_expiry.py
@@ -4,16 +4,17 @@ Counter for the days till a HTTPS (TLS) certificate will expire.
 For more details about this sensor please refer to the documentation at
 https://home-assistant.io/components/sensor.cert_expiry/
 """
-import datetime
 import logging
 import socket
 import ssl
+from datetime import datetime, timedelta
 
 import voluptuous as vol
 
 import homeassistant.helpers.config_validation as cv
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_NAME, CONF_HOST, CONF_PORT)
+from homeassistant.const import (CONF_NAME, CONF_HOST, CONF_PORT,
+                                 EVENT_HOMEASSISTANT_START)
 from homeassistant.helpers.entity import Entity
 
 _LOGGER = logging.getLogger(__name__)
@@ -21,7 +22,7 @@ _LOGGER = logging.getLogger(__name__)
 DEFAULT_NAME = 'SSL Certificate Expiry'
 DEFAULT_PORT = 443
 
-SCAN_INTERVAL = datetime.timedelta(hours=12)
+SCAN_INTERVAL = timedelta(hours=12)
 
 TIMEOUT = 10.0
 
@@ -34,11 +35,20 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
 
 def setup_platform(hass, config, add_devices, discovery_info=None):
     """Set up certificate expiry sensor."""
-    server_name = config.get(CONF_HOST)
-    server_port = config.get(CONF_PORT)
-    sensor_name = config.get(CONF_NAME)
+    def run_setup(event):
+        """Wait until Home Assistant is fully initialized before creating.
 
-    add_devices([SSLCertificate(sensor_name, server_name, server_port)], True)
+        Delay the setup until Home Assistant is fully initialized.
+        """
+        server_name = config.get(CONF_HOST)
+        server_port = config.get(CONF_PORT)
+        sensor_name = config.get(CONF_NAME)
+
+        add_devices([SSLCertificate(sensor_name, server_name, server_port)],
+                    True)
+
+    # To allow checking of the HA certificate we must first be running.
+    hass.bus.listen_once(EVENT_HOMEASSISTANT_START, run_setup)
 
 
 class SSLCertificate(Entity):
@@ -97,6 +107,6 @@ class SSLCertificate(Entity):
             return
 
         ts_seconds = ssl.cert_time_to_seconds(cert['notAfter'])
-        timestamp = datetime.datetime.fromtimestamp(ts_seconds)
-        expiry = timestamp - datetime.datetime.today()
+        timestamp = datetime.fromtimestamp(ts_seconds)
+        expiry = timestamp - datetime.today()
         self._state = expiry.days