Generally Date Params default to current date.
Thus you can have one param and two formula.
@Startdate
date(year(dateadd("m", -1, {?dateparam}), month(dateadd("m", -1, {?dateparam}), 01)
@EndDate
date(year({?dateparam}), month({?dateparam}), 01)-1
Of course for these formula you do not really need a param and can just use the Crystal function currentdate instead.
Use these formula in select and display on report. Crystal will calculate and pass to database.
Ian