AddLines not working as expected

[Jay Schille]

Here's what I've got going:
(EventDate is a date field in the database)

local oDate
     oDate=date("04/15/08")
        if HowOften="monthly"
             AddLines EventDate, monthmath(oDate, 1), monthmath(oDate, 
8), "M", 1
         endif

This produces eight new records, the first of which, as expected, has 
the EventDate of "05/15/08." But the next seven records have 
EventDates  sequenced as "06/01/08," "07/01/08"... Not what I want.

Is this a bug or a feature or am I not using the procedure correctly?
-- 
Jay Schille