Hi barrowb,
As far as I know, the GETPIVOTDATA worksheet function will only return a value from a single cell in your PivotTable.
You could consider...
1. Using a calculated field or calculated item to display the current month plus back-orders value
2. Summing multiple GETPIVOTDATA() expressions (if the list of dates is short).
3. Using lookup functions like Index-Match to find and sum those values.
4. Using a query of your source data that filters just those records that meet your criteria.