Additional day on payment terms (Net 31)

After saving a new invoice the Net terms change to show an additional day; so if we've selected terms of Net 30 days it changes to become Net 31, 45 days becomes 46 etc... No matter what I do to go back and double-check or re-select the correct terms it still always shows up on the final invoice with an extra day. We need to resolve this as obviously we want to make sure the correct terms are on our invoices!

Is this a bug or something we're doing wrong?

Thanks

P.S. very pleased with Zoho Invoice apart from this little annoyance :-)