writing to excel based on a value

i have this, as a multi row formula, which sets a new field "newfield" :   if abs(DateTimeDiff([Row+1:EDWTD02_DOC_ISSUE_Y],[EDWTD02_DOC_ISSUE_Y],"days"))>400 then "Y" else "N" endif
//take the difference of the dates for each unique key_fld if there is more than one, in the table. if the ABS(difference) is more than 400, write out the key_fld. in theory, time moves forward and you don't need the ABS.

now I just want to write out, to excel, where newfield="Y", the key field.  i tried doing a formula, creating the field "PO_w_400_days_btn" as a string with this:

if [NewField]="Y"
then [key_fld]
else "NULL"
endif

but is that necessary or can you write an excel field based on testing if "newfield"="Y" without the formula command?

thanks

becki kain