UNIX for Beginners Questions & Answers

$ set -- FN 5 8
$ shift
$ awk -F, -v"FLDS=$*" '
BEGIN                   {FCNT = split(FLDS, FLD, " ")
                        }

function CV(IX)         {TMP = (19+$(IX-1))*1E6 + $(IX)
                         Y   = int(TMP/1E4)
                         return sprintf ("%d-%02d-%02d", Y, int(TMP%Y/100), TMP%100)
                        }

                        {for (i=1; i<=FCNT; i++) $FLD[i] = $FLD[i] ? CV(FLD[i]) : "0001-01-01"  
                        }
1
' OFS=, file
29,A Store,A Street,1,2011-12-13,aaaa,0,1989-12-13
30,B Store,B Street,0,1999-12-13,aaaa,1,2006-12-13
31,C Store,C Street,1,2005-12-13,aaaaa,1,2008-12-13
32,D Store,D Street,0,0001-01-01,aaaa,1,2015-03-23
33,E Store,E Street,1,2012-12-12,bbbb,0,0001-01-01
34,F Store,F Street,1,2010-12-12,cccc,0,1997-12-12