add element by expression : how is it possible to add days to date parameter

HI,

in data model  "add element by expression"

i would like to create an expression like :  "week  from  04/02/2013 to 08/02/2013"

my date parameter is DAT_PARAM  and it's a date and its format is DD/MM/YYYY

I tries this but without result

CONCAT('Week from  ',  CONCAT( FORMAT_DATE(DAT_PARAM,'dd/MM/yyyy'),  CONCAT('  to  ', FORMAT_DATE(DAT_PARAM + 4,'dd/MM/yyyy'))))

i have searched on web and in oracle documentation but no example

this expression return : week from 04/02/2013 to 04/02/2013

DAT_PARAM+4 is unknow

Is it possible ??

regards