Calculated field - CDS

Hi Experts,

I'm using the CDS in hana studio, I need to have in final report a custom field ZFIELD like this:

IF budat => sy-datum AND budat =< (sy-datum + 30). ZFIELD = '+30D'

ELSEIF budat < sy-datum AND budat => (sy-datum - 30). ZFIELD = '-30D'

How can I have?

Thanks