Search API / Facets: How can I define a range / manipulate the search query in D8?
Afterwards here my solution: I've solved it with condition groups in the hook_search_api_query_alter() function:

function MYMODULE_search_api_query_alter($query) {
    //...

    $conditionGroup = $query-&gt;createConditionGroup('AND');
    $conditionGroup-&gt;addCondition('created', $firstDayOfYear, '&gt;=');
    $conditionGroup-&gt;addCondition('created', $lastDayOfYear, '&lt;=');
    $query-&gt;addConditionGroup($conditionGroup);

    //...
}