home-assistant.github.io/blog/2015/05/09/utc-time-zone-awareness/index.html
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<h1 class="title indent">UTC & Time zone awareness</h1>
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<time datetime="2015-05-09T23:08:00+00:00" pubdate data-updated="true"><i class="icon-calendar"></i> May 9, 2015</time>
<span class="byline author vcard"><i class='icon-user'></i> Paulus Schoutsen</span>
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<p>I have recently merged code to refactor Home Assistant to use only UTC times internally. A much needed refactor. Ive 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.</p>
<p>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 <code>homeassistant.util.dt.utcnow()</code>. This is a timezone aware UTC datetime object. <a href="https://github.com/balloob/home-assistant/blob/dev/homeassistant/util/dt.py"><code>homeassistant.util.dt</code></a> is a new util package with date helpers.</p>
<p>There is also such a thing as local time. Local time is based on the time zone that you have setup in your <code>configuration.yaml</code>. Local times should only be used for user facing information: logs, frontend and automation settings in <code>configuration.yaml</code>.</p>
<h3>Setting up your time zone</h3>
<p>Setting up a time zone happens in <code>configuration.yaml</code>. If you have no time zone setup, it will be auto detected using the existing detection code using <a href="https://freegeoip.net">freegeoip.net</a>. You can find a list of compatible time zones on <a href="http://en.wikipedia.org/wiki/List_of_tz_database_time_zones">Wikipedia</a>.</p>
<div class="highlighter-coderay"><div class="CodeRay">
<div class="code"><pre><span class="key">homeassistant</span>:
<span class="key">time_zone</span>: <span class="string"><span class="content">America/Los_Angeles</span></span>
</pre></div>
</div>
</div>
<h3>Compatibility</h3>
<p>The changes to the code are mostly backwards compatible. The old <code>hass.track_time_change</code> and <code>hass.track_point_in_time</code> use now internally two new methods: <code>hass.track_utc_time_change</code> and <code>hass.track_point_in_utc_time</code>. The usage of the old methods have not changed and should be backwards compatible.</p>
<p>This refactor adds a new migration for the database adding a <code>utc_offset</code> 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.</p>
<h3><a class="title-link" name="backwards-incompatible-stuff" href="#backwards-incompatible-stuff"></a> Backwards incompatible stuff</h3>
<p>All built-in components have been upgraded. The following list is only for people that run custom components:</p>
<ul>
<li><code>hass.track_time_change</code> and <code>hass.track_point_in_time</code> will now return a time zone aware datetime object. Python does not allow comparing a naive with an aware datetime object.</li>
<li>the sun attributes for rising and setting are now in UTC. The methods <code>sun.next_rising(hass)</code> and <code>sun.next_setting(hass)</code> are backwards compatible, just be careful if you used to read the raw attributes.</li>
<li>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.</li>
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