OTBI - prefix for current date minus days

Summary

How can I define prompt for an OTBI report which is the current date minus variable of days

Content

Hi,

I'm trying to create a prompt of number, so if the user will insert for example '20', the report will be effective for the current_date-20.

I'm trying to insert a prefix like this: SET VARIABLE PARAM_EFFECTIVE_DATE = '@{system.currentTime} -@{DaysMin}[YYYY-MM-DD]';

(DaysMin the a prompt var).

Unfortunately it does not work. 

Do you have any suggestion?

Version

R13 18c