A quarterly update is a summary of your income and expenses for a three-month period, sent to HMRC from your software. It is not a tax return and not a payment demand — it is totals by category, nothing more. The standard quarters end on 5 July, 5 October, 5 January and 5 April, with deadlines on the 7th of the following month: 7 August, 7 November, 7 February and 7 May. You can elect for calendar quarters ending 30 June, 30 September, 31 December and 31 March instead, which most people find easier to reconcile against bank statements. Crucially, quarterly updates do not have to be perfect. They are provisional, you can correct them later, and nothing is finalised until your end-of-year declaration.