Date field select option in custom module
I can't find a link to any documentation for this at the moment, but the default value for a date field expects the date in a certain format: yyyy-mm-dd. It might sound counter-intuitive but something like this should work:

// Make up a date with the correct year
$time = mktime(0, 0, 0, 1, 1, $year);

// Format the date
$default_value = date('Y-m-d', $time);

// Set the default value
$form['release_date']['#default_value'] = $default_value;