How add a function in a condition method in db api
I believe the addExpression is for SELECT clause expressions

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Database%21Query%21Select.php/function/Select%3A%3AaddExpression/8.2.x

For a WHERE clause expression it should be 

$query-&gt;where('UNIX_TIMESTAMP(et.field_symposium_event_time_value) BETWEEN :start AND :end', array(':start' =&gt; $day_start, ':end' =&gt; $day_end));


https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Database%21Query%21QueryConditionTrait.php/function/QueryConditionTrait%3A%3Awhere/8.2.x

And then of course remove your $query-&gt;condition(...); line.