title: If "min_index_age" is determined using the index creation time as reference what is the significance of @timestamp field in the datastream[Opensearch]
body: <p>I am currently migrating some timeseries data stored in a regular index to a datastream managed using ISM policies. The policy definition is provided below</p>
  <pre><code>{
      &quot;policy&quot;: {
          &quot;policy_id&quot;: &quot;policy1&quot;,
          &quot;description&quot;: &quot;policy for ingesting sensor data&quot;,
          &quot;default_state&quot;: &quot;hot&quot;,
          &quot;states&quot;: [{
                  &quot;name&quot;: &quot;hot&quot;,
                  &quot;actions&quot;: [{
                      &quot;rollover&quot;: {
                          &quot;min_size&quot;: &quot;32gb&quot;
                      }
                  }],
                  &quot;transitions&quot;: [{
                      &quot;state_name&quot;: &quot;delete&quot;,
                      &quot;conditions&quot;: {
                          &quot;min_index_age&quot;: &quot;30d&quot;
                      }
                  }]
              },
              {
                  &quot;name&quot;: &quot;delete&quot;,
                  &quot;actions&quot;: [{
                      &quot;delete&quot;: {}
                  }],
                  &quot;transitions&quot;: []
              }
          ],
          &quot;ism_template&quot;: [{
              &quot;index_patterns&quot;: [
                  &quot;sensordata_*_history*&quot;
              ],
              &quot;priority&quot;: 100
          }]
      }
  }
  </code></pre>
  <p>I would have assumed that the deletion would happen based on the @timestamp field, i.e. if an index contains all data that is older than 30d, then the index would be deleted.</p>
  <p>But it seems the index creation time is used as the reference time, then what is the purpose of the @timestamp field in the datastream ?</p>