CIS runs on tax months from the 6th to the 5th, not calendar months. Here's why that matters and how to avoid grouping payments incorrectly.
10 June 2026
If you've ever looked at your CIS paperwork and wondered why a payment made on the 3rd of the month doesn't seem to belong where you'd expect, the answer is almost always the same: CIS doesn't run on calendar months. It runs on tax months, and getting this straight early saves a lot of confusion later.
A CIS tax month runs from the 6th of one calendar month to the 5th of the next. So the tax month you'd probably call "June" for CIS purposes actually runs from 6 June to 5 July. This isn't a CIS quirk invented in isolation, it follows the same 6th-to-5th convention used for PAYE tax months and the wider UK tax year, which itself runs 6 April to 5 April.
Most business software and habits default to the calendar month. Invoices, payroll runs, and general bookkeeping usually think in terms of "June" meaning 1 to 30 June. CIS doesn't work that way, so a payment made on 3 June actually falls into the tax month that started on 6 May, not the one most people would instinctively call June.
This matters because everything downstream depends on which tax month a payment belongs to:
Get the tax month wrong and you can end up issuing a statement or return that doesn't match what HMRC expects to see, even though every individual figure on it is correct.
Say you pay a subcontractor on 4 July. That payment falls in the tax month running 6 June to 5 July, not the "July" tax month, which only starts on 6 July. If you'd paid them two days later, on 6 July, it would belong to the next tax month entirely, 6 July to 5 August. Two payments four days apart, and they can land in different tax months and therefore need to appear on different statements.
This is exactly the kind of edge case that trips up trades with payments bunched near a month boundary. It's a particular headache for subcontracted crews paid around weather delays or job completion dates, where payments can cluster right on the 5th or 6th without anyone planning it that way.
Because the 14-day rule for Payment and Deduction Statements and the 19th-of-the-month CIS300 deadline are both counted from the end of the relevant tax month, not the calendar month, it's worth working out the tax month boundaries for the whole year in advance rather than recalculating them each time. A simple table of tax month start and end dates, or software that handles it automatically, removes the guesswork.
If you pay the same subcontractor more than once within a single tax month, whether that's a weekly payment run or several invoices settled together, those payments should be consolidated onto one Payment and Deduction Statement for that tax month, not issued as separate statements. Conversely, if a subcontractor is paid either side of a tax month boundary, even a few days apart, those payments need to go on separate statements for separate tax months. Our CIS300 monthly return guide covers how these figures then roll up into your monthly HMRC filing.
Once you know the tax month cut-off dates, the safest habit is to check every payment date against the 6th-to-5th boundary before you group it, rather than assuming "this month's payments" means the calendar month. It sounds like a small distinction, but it's the single most common reason a contractor's statements and CIS300 return don't line up.
If you'd rather not track tax month boundaries by hand every time you run payments, our CIS statement generator groups payments into the correct 6th-to-5th tax month automatically from the payment dates you give it, so a payment on the 4th and one on the 7th are never accidentally lumped together.
See our guides hub for more on deduction rates, verification, and filing deadlines.