UTC & Time zone awareness

I have recently merged code to refactor Home Assistant to use only UTC times internally. A much needed refactor. I’ve added some extra test coverage to time sensitive parts to ensure stability. The code has been live in the dev branch for the last 9 days and will be soon released to the master branch.

From now on all internal communication will be done in UTC: time changed events, datetime attributes of states, etc. To get the current time in UTC you can call homeassistant.util.dt.utcnow(). This is a timezone aware UTC datetime object. homeassistant.util.dt is a new util package with date helpers.

There is also such a thing as local time. Local time is based on the time zone that you have setup in your configuration.yaml. Local times should only be used for user facing information: logs, frontend and automation settings in configuration.yaml.

Setting up your time zone

Setting up a time zone happens in configuration.yaml. If you have no time zone setup, it will be auto detected using the existing detection code using freegeoip.net. You can find a list of compatible time zones on Wikipedia.

homeassistant:
  time_zone: America/Los_Angeles

Compatibility

The changes to the code are mostly backwards compatible. The old hass.track_time_change and hass.track_point_in_time use now internally two new methods: hass.track_utc_time_change and hass.track_point_in_utc_time. The usage of the old methods have not changed and should be backwards compatible.

This refactor adds a new migration for the database adding a utc_offset column to events and states. This information is currently not used but can prove useful in the future when we start analyzing the historical data.

Backwards incompatible stuff

All built-in components have been upgraded. The following list is only for people that run custom components:

• hass.track_time_change and hass.track_point_in_time will now return a time zone aware datetime object. Python does not allow comparing a naive with an aware datetime object.
• the sun attributes for rising and setting are now in UTC. The methods sun.next_rising(hass) and sun.next_setting(hass) are backwards compatible, just be careful if you used to read the raw attributes.
• the API sends all times in UTC. If you use anything else besides the frontend to talk to HA, make sure it handles it differently.