In the post there is no restriction that we should not use SQL statements, so I assumed that we can use SQL statements in the program.
Read the file and move the date to the vairable DATE-FROM-INP-FILE and execute the below SQL code in your COBOL program.
SELECT (DATE(:DATE-FROM-INP-FILE) + 6 MONTH), (DATE(:DATE-FROM-INP-FILE) - 6 MONTH)
INTO :DATE-PLUS-SIX-MON, :DATE-MINUS-SIX-MON
Post if you have any questions.