First, look up the name of the field you need in the schema browser (http://localhost:8983/solr/admin/ for a local install). You should be able to locate it based on the machine name of your Drupal field (assuming the index is up-to-date).
hook_apachesolr_query_alter()
Next, drop this hook in a custom module (note this is for Drupal 7). Substitute your field's name in for dm_field_date.
/** * Implements hook_apachesolr_query_alter(). */ function hook_search_apachesolr_query_alter(DrupalSolrQueryInterface $query) { $query->addParam('bf', array('freshness' => 'recip(abs(ms(NOW/HOUR,dm_field_date)),3.16e-11,1,.1)' )); }
Fine-tuning
This function will apply a +10 boost to a document with today's date, tapering off to about +1 after a year. To write your own boost function, consult the Solr Wiki page on FunctionQuery. Note there are no spaces in the function queries! Choosing the constants can feel a bit blind, so visualizing the function on Wolfram Alpha helps (the x-axis is in milliseconds):
