Create Alert prompt for expiry date less than 30days from current date

Hi,

I got a field due_date, what formula should I use to create a alert which will prompt out and tell me my product will expiry with 30days.

if {abc.due_date} < 30 from CurrentDate

then ???

I tried to search from interent but it is not easy to find a tutorial on creating alert in crystal report.

Thanks.