user: Question: When was the build date for FF20 PRR class and Erie Built builder's model?
Columns: ['PRR Class', 'Builder’s Model', 'Build date', 'Total produced', 'Wheel arrangement', 'Service', 'Power output']
Types: ['text', 'text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['FS10', 'H10-44', '1948–1949', 55.0, 'B-B', 'Switcher', 'hp (kW)']
SQL:

assistant: SELECT "Build date" FROM "Table" WHERE "PRR Class" = 'FF20' AND "Builder’s Model" = 'Erie Built';