Does this do what you want?
Code:
program:
mynum = strcat($$#completiondate, 'd');
date_arithmetic(today(),mynum)
EDIT: This'll probably fall over if you have nothing in completiondate, an if-then should help:
Code:
program:
if
!$$#completiondate
then ''
else
mynum = strcat($$#completiondate, 'd');
date_arithmetic(today(),mynum)
fi