So if the user selects August the report should run from September 30, 2000 to September 30, 2001?
You've already got how to print the date range, so I'm guessing you need to be able to get the end of the following month.
With VFP >=SP3, I'd use the new capabilities of the DATE() command. e.g.,
In this case, I am assuming that year is always the current year, but I have included an alternate in case it isn't...

lnMonth = thisform.spnMonth.Value
** if date: lnMonth = MONTH(thisform.txtDate.Value)
lnYear = YEAR(DATE())
** if date: YEAR(thisform.txtDate.Value)

ldStart = GOMONTH(DATE(lnYear, lnMonth, 1),2)-1
ldEnd = GOMONTH(DATE(lnYear+1, lnMonth, 1),2)-1

With VFP before sp3:

lnMonth = thisform.spnMonth.Value
** if date: lnMonth = MONTH(thisform.txtDate.Value)
lnYear = YEAR(DATE())
** if date: YEAR(thisform.txtDate.Value)

ldStart = GOMONTH(CTOD(ALLTRIM(STR(lnMonth))+"/01/"+ALLTRIM(STR(lnYear))),2)-1
ldEnd = GOMONTH(CTOD(ALLTRIM(STR(lnMonth))+"/01/"+ALLTRIM(STR(lnYear+1))),2)-1