CREATE TABLE table_56698 ("PRR Class" TEXT, "Builderpars_m_singles Model" TEXT, "Build date" TEXT, "Total produced" FLOAT, "Wheel arrangement" TEXT, "Service" TEXT, "Power output" TEXT)

When was the build date for ff20 PRR class and erie built builder's model?
SELECT "Build date" FROM table_56698 WHERE "PRR Class" = 'ff20' AND "Builder\u2019s Model" = 'erie built'
SELECT "Build date" FROM "table_56698" WHERE "Builder\u2019s Model" = 'erie built' AND "PRR Class" = 'ff20'