Allows to override the reporting start date.
The event handler should return the new reporting start date as string. The default reporting start date
matches the first order date.
Example:
public function subscribeEvents()
{
Backend::$events->addEvent('shop:onGetStartReportingDate', $this, 'get_start_reporting_date');
}
public function get_start_reporting_date($default_date)
{
return '2009-10-01';
}