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If you need to have your VAT returns in March, June and September, you will need to set up your VAT so that the Start of current VAT period is showing as October. This will set up the quarters so that you can file in December, March, June and September going forwards.
When you then go to the Submit Return for your first filing within QuickBooks Online, you will be able change the Start Date of the filing period for this filing only. This will allow you to have the five month filing from 01/08/2017 - 31/12/2017 as required.
Please note, this extending of the date range can only be done on the very first filing within QuickBooks Online. Once you file your first return, the start date of subsequent filings will be set as the day after your previous filing.
Going forwards, QuickBooks will then automatically select the quarter dates based on the filing previously and the settings entered during VAT set up.
Let me know if you have any queries. Or, if you'd like to go through this over the phone, you can call our dedicated team who will be happy to assist you. Our contact details can be found here along with our opening hours.