/**
* Every operation needs material to start.
* @returns the time when material of this operation is available
*/
public DueTime GetEarliestPossibleStartTime()
{
IAggregator aggregator = ZppConfiguration.CacheManager.GetAggregator();
DueTime earliestDueTime =
aggregator.GetEarliestPossibleStartTimeOf(
this);
return(earliestDueTime);
}