Can you post a sample file for us to tinker with?
I believe in this reference you can just concatenate a new field as the breakfield.
calc_breakfield, unstored calculation = MANUFACTURER NUMBER & Year(date field)
GetSummary ( SummaryQtysum ; calc_breakfield )
What are you trying to accomplish in real-life terms? I ask because out of dozens of complete solutions I've developed, I've used GetSummary() maybe twice.
Are you sure you need it?
This is unnecessary.
If you sort by both fields as stated but make the break field z_year in date it will summarise correctly.