title: How to select time_key from json log with fluentd?
body: <p>I having some logs from the cluster which have json format.</p>
  <pre class="lang-xml prettyprint-override"><code>&lt;filter mylog.**&gt;
      @type parser
      format json
      key_name json_part
      reserve_data true
      remove_key_name_field true
  &lt;/filter&gt;
  </code></pre>
  <p>Now the idea is the inside that json I have a field let's say <code>my_time</code> which I want to be used as time filter in Kibana.</p>
  <p>The problem that I saw is the following, if the time format is in <code>ISO8601</code> then in Kibana I see the field as type <code>date</code> and I can select it as <code>time_field</code> when creating the index. But not every time! Sometimes it simply gets it as string.</p>
  <p>And for other applications I expect to have maybe a different time format, therefore I want some control over it.</p>
  <pre class="lang-xml prettyprint-override"><code>time_key my_time
  time_format ...
  </code></pre>
  <p>However I tried it once like this and it did not work.</p>
  <pre class="lang-xml prettyprint-override"><code>&lt;filter mylog.**&gt;
      @type parser
      format json
      time_key my_time   &lt;--- added this here
      key_name json_part
      reserve_data true
      remove_key_name_field true
  &lt;/filter&gt;
  </code></pre>
  <p>So with ISO8601 I also have inconsistencies, sometimes interpreted as date sometimes as string.
  And if I add other formats I'm not sure how to properly handle them.</p>
  <p>What is the right way to make sure your field from <code>json</code> which represents the time gets send well as date type to Kibana?</p>