/**
* A private method to calculate the operation interval start date at the
* upper bound of a range of operation intervals that require a ZIF update,
* where the upper bound has been set back by the required number of
* operation intervals so that current trends in differences between
* forecast and actual delivery can be calculated.
*
* @access private
* @param PEAR::Date $oDate The start date of the operation interval at the
* upper bound of the operation interval range
* requiring a ZIF update.
* @return PEAR::Date The new upper bound date.
*/
function _getTrendUpperDate($oDate)
{
$seconds = ZONE_FORECAST_TREND_OFFSET * OX_OperationInterval::secondsPerOperationInterval();
$oDate->subtractSeconds($seconds);
return $oDate;
}
