>how can I add one month to the date?

Use the GOMONTH function. But note that it has some "features" -- see below:

ldDate = {^2009-01-30}
?GOMONTH(ldDate,1)                  && Returns 02/28/2009
?GOMONTH(ldDate,2)                  && Returns 03/30/2009

ldDate = GOMONTH(ldDate,1)    && Sets date to 02/28/2009
ldDate = GOMONTH(ldDate,1)    && Sets date to 03/28/2009; this has now lost 2 days from the above

In the above the GOMONTH function moves the current date ahead one month; but if the next month does not have the day number then the last day number is used. If you are always wanting a fixed day of the month, then be aware of the effect for dates after 28 for Feb and the other months that only have 30 instead of 31.