Manually changing the VAT quarter period isn't possible, theracefarm.
The August 28 date that you're seeing may be the filing due date. Since this is set up and filed with HMRC, we recommended reaching out to them. Thus, QuickBooks Online (QBO) only receives VAT return obligations from the HMRC, and we're unable to alter this in QBO.
You can click the Reply button if you still need help with VAT. We'll ensure your needs are taken care of, theracefarm.
