How to boost relevance of Search API solr with creation date?
To do this (Drupal 7), one needs to implement the below hook:

hook_search_api_solr_query_alter(&amp;$call_args, SearchApiQueryInterface $query)


This hook can go in the theme or module layer.

Example code could be:

/**
 * Implements hook_search_api_solr_query_alter().
 *
 * @see https://www.drupal.org/node/2009760#find-out-field-id
 * @see https://www.drupal.org/project/search_api_solr/issues/2194055
 * @see https://www.metaltoad.com/blog/date-boosting-solr-drupal-search-results
 * @see https://www.drupal.org/forum/support/module-development-and-code-questions/2014-12-18/extending-search-api-solr-search-with
 * @see https://www.drupal.org/project/search_api_solr/issues/2392499
 * @see https://www.drupal.org/project/search_api/issues/1694698
 * @see https://books.google.com.au/books?id=othpBgAAQBAJ&amp;pg=PA157&amp;lpg=PA157&amp;dq=3.16e-11+solr
 */
function mytheme_search_api_solr_query_alter(&amp;$call_args, SearchApiQueryInterface $query) {
  // Boost more recently created content. This is taken directly from an
  // apachesolr query with +1 boost (+1 for every 100).
  $fields_names = $query-&gt;getIndex()-&gt;server()-&gt;getFieldNames($query-&gt;getIndex());
  if (isset($fields_names['created'])) {
    // If using node property 'created', get solr name of it  e.g. 'dm_created'.
    $created_field_name = $fields_names['created'];
    // @codingStandardsIgnoreStart
    // Field name version.
    // $created_field_name = $fields_names['field_date'];
    // @codingStandardsIgnoreEnd
    // Ensure '$call_args['params']['bf']' exists.
    if (!isset($call_args['params']['bf'])) {
      $call_args['params']['bf'] = [];
    }
    $call_args['params']['bf'][] = "recip(ms(NOW,{$created_field_name}),3.16e-11,0.5,0.5)^100.0";
  }
}