Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_56698 ( "PRR Class" text, "Builder\u2019s Model" text, "Build date" text, "Total produced" real, "Wheel arrangement" text, "Service" text, "Power output" text )

### Question ###
When was the build date for ff20 PRR class and erie built builder's model?

### Accurate SQL ###

SELECT "Build date" FROM table_56698 WHERE "PRR Class" = 'ff20' AND "Builder\u2019s Model" = 'erie built'