Set default value for input_datetime (#21919)

* Set default value for input_datetime

If no initial value is set and no value is available to be restored, set the default value as specified in the docs to 1970-01-01 00:00.

* Use regular if statement

Ternary statements can be tricky if you try to keep the value the same if not something

* Add test for default values

Check that if no initial value is set, state returns 1970-01-01 at 00:00

* Fix tests - was passing wrong args to time/date

* Verify we get a timestamp attribute for input_datetime

This adds a check that when using the default timestamp of 1970-1-1 00:00:00, we
get a timestamp attribute. This is waht prompted this PR in the first place, as
when specifying an automation trying to access the timestamp attribute for a non-
initialized input_datetime HASS wouldn't start.

* Simplify the change for a default value

Based on @balloob comment. Simplifying the code

* Revert "Simplify the change for a default value"

This reverts commit c2d67f19a6.

tests/components/input_datetime/test_init.py

@@ -199,6 +199,39 @@ def test_restore_state(hass):
     assert state_bogus.state == str(initial)
 
 
+@asyncio.coroutine
+def test_default_value(hass):
+    """Test default value if none has been set via inital or restore state."""
+    yield from async_setup_component(hass, DOMAIN, {
+        DOMAIN: {
+            'test_time': {
+                'has_time': True,
+                'has_date': False
+            },
+            'test_date': {
+                'has_time': False,
+                'has_date': True
+            },
+            'test_datetime': {
+                'has_time': True,
+                'has_date': True
+            },
+        }})
+
+    dt_obj = datetime.datetime(1970, 1, 1, 0, 0)
+    state_time = hass.states.get('input_datetime.test_time')
+    assert state_time.state == str(dt_obj.time())
+    assert state_time.attributes.get('timestamp') is not None
+
+    state_date = hass.states.get('input_datetime.test_date')
+    assert state_date.state == str(dt_obj.date())
+    assert state_date.attributes.get('timestamp') is not None
+
+    state_datetime = hass.states.get('input_datetime.test_datetime')
+    assert state_datetime.state == str(dt_obj)
+    assert state_datetime.attributes.get('timestamp') is not None
+
+
 async def test_input_datetime_context(hass, hass_admin_user):
     """Test that input_datetime context works."""
     assert await async_setup_component(hass, 'input_datetime', {