Input datetime (#3390)

* First documentation

* Add documentation for input_datetime

source/_components/input_datetime.markdown

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+---
+layout: page
+title: "Input Datetime"
+description: "Instructions how to integrate the Input Datetime component into Home Assistant."
+date: 2017-09-14 16:01
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Automation
+ha_release: TODO
+---
+
+The `input_datetime` component allows the user to define date and time values that can be controlled via the frontend and can be used within automations and templates.
+
+To add three datetime inputs to your installation, one with both date and time, and one with date or time each, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+input_datetime:
+  both_date_and_time:
+    name: Input with both date and time
+    has_date: true
+    has_time: true
+  only_date:
+    name: Input with only date
+    has_date: true
+    has_time: false
+  only_time:
+    name: Input with only time
+    has_date: false
+    has_time: true
+```
+
+Configuration variables:
+
+- **[alias]** (*Required*): Alias for the datetime input. Multiple entries are allowed.
+  - **name** (*Optional*): Friendly name of the datetime input.
+  - **has_time** (*Optional*): Set to `true` if this input should have time. Defaults to `false`.
+  - **has_date** (*Optional*): Set to `true` if this input should have a date. Defaults to `false`.
+  - **initial** (*Optional*): Set the initial value of this input. Defaults to '1970-01-01 00:00'.
+
+A datetime input entity's state exports several attributes that can be useful in automations and templates:
+
+- **has_time**: `true` if this entity has time.
+- **has_date**: `true` if this entity has a date.
+- **year**, **month**, **day** (Only available if *has_date* is true): The year, month and day of the date.
+- **hour**, **minute**, **second** (Only available if *has_time* is true): The hour, minute and second of the time.
+- **timestamp**: A timestamp representing the time held in the input. If *has_date* is true, this is the UNIX timestamp of the date / time held by the input. Otherwise (i.e., if only *has_time* is true) the number of seconds since midnight representing the time held by the input.