I am trying to work out 30 days from the end of the month. The problem I am having however is that some are returning the 29th day of the month instead of the 30th.

Date(DateAdd('m',1{ARInvoice.InvoiceDate})-day(({ARInvoice.InvoiceDate}))+tonumber({ARTerms.T ermDays}))

So for example i have the date 15/06/2011 i start by taking the 15 days off back to the start of the month. I then add a month plus 30 days as the term days in this case is 30 days. I want to go to the end of the month and then add 30 days starting from the first of the next month.

Current formula as shown above is returning the following:
Terms InvoiceNo CurrentBalance InvoiceDate DueDate
30 00431837 2,929.72 31/05/2011 29/06/2011 Incorrect (should be 30/06/2011)
30 00431253 2,974.05 19/05/2011 30/06/2011 Correct (30 days from start of next month)

I don't understand why it is correct for most results but a day out on other results. Any help with this problem would be greatly appreciated