user: Question: What is the build date for PRR Class GF30a?
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: ['GS4', '44 Ton', '1948–1950', 46.0, 'B-B', 'Switcher', 'hp (kW)']
SQL:

assistant: SELECT "Build date" FROM "Table" WHERE "PRR Class" = 'GF30a';