Written by Employment Tribunal – early conciliation – limitation arithmetic
The Acas early conciliation period can now run to twelve weeks, double the six weeks that applied before December 2025. What that changes is not the obligation to notify Acas but the arithmetic of the deadline itself. This article computes the outcomes for three fact patterns – unfair dismissal, discrimination and unpaid wages – and shows why the “at least one month left” rule is doing more work than ever.
Snapshot
Since 1 December 2025 the conciliation officer has had up to twelve weeks, rather than six, to promote a settlement, with no power to extend that period; the change applies to every early conciliation notified to Acas on or after that date. Because the tribunal clock stops between Day A and Day B, a longer conciliation period means a longer stop, and a final deadline that can land months after the date a claimant first calculated. The date that decides a claim is not the one on the dismissal letter; it is the date the arithmetic produces.
Two clocks, not one
Every tribunal claim runs on two clocks at once. The first is the basic limitation clock: for most claims the claim must be presented before the end of the period of three months beginning with the date of the act, the dismissal or the underpayment – three months less a day, in the usual shorthand. The second clock is early conciliation. Before a prospective claimant can present most tribunal claims, they must provide the prescribed information to Acas, and they may not present the claim without an early conciliation certificate. The certificate is a gateway, not a merits filter. Acas cannot represent the claimant, take sides, help prepare the case or say whether the claim is valid. Conciliation buys talking time; it does not plead the claim.
The two clocks interact through two dates the legislation names Day A and Day B, and that interaction decides when a claim must actually be presented. Twelve weeks of conciliation is twelve weeks in which the limitation clock is stopped.
Day A, Day B and the one-month rule
Day A is the day the claimant complies with the requirement to contact Acas; Day B is the day they receive, or are treated as receiving, the early conciliation certificate. In working out when the time limit expires, the period beginning with the day after Day A and ending with Day B is not counted. And if the time limit would expire during the period beginning with Day A and ending one month after Day B, it expires instead at the end of that period – one month after Day B. That second limb is the “at least one month left” rule: a claimant who notified Acas in time always has at least a full month after the certificate to present the claim.
Together the limbs give a usable rule of thumb. The final deadline is the later of two dates: the original deadline shifted forward by the number of days between Day A and Day B, and one month after Day B. Two cautions attach. A certificate sent by email is treated as received on the day it is sent; a posted certificate is treated as received on the day it would arrive in the ordinary course of the post. And the twelve weeks is a maximum, not an entitlement: the clock stops only for the days that actually elapse between Day A and Day B. A certificate that arrives in week three means three weeks gained, not twelve.
Worked example: unfair dismissal
An unfair dismissal complaint must be presented before the end of the period of three months beginning with the effective date of termination, and the Day A and Day B extension applies to that limit. Take a dismissal with an effective date of termination of 10 July 2026: the basic limit expires at the end of 9 October 2026. The claimant notifies Acas on 14 September 2026 – Day A – with 25 days to spare, and conciliation runs the full twelve weeks; the certificate is issued and emailed on 7 December 2026 – Day B.
| Milestone | Date | Why |
|---|---|---|
| Basic limit | 9 October 2026 | Three months from the effective date of termination |
| Day A | 14 September 2026 | Acas notified with 25 days left |
| Day B | 7 December 2026 | Certificate after the full twelve weeks |
| Shifted deadline | 1 January 2027 | Basic limit plus the 84 paused days |
| One-month floor | 7 January 2027 | One month after Day B |
| Final deadline | 7 January 2027 | The later of the two |
Notice which limb did the final work: the pause alone carried the deadline to 1 January 2027; the floor pushed it six days further because the shifted date fell inside the month after Day B. A claimant who notified with less than a month to spare ends with a new-year deadline.
Worked example: discrimination
A discrimination complaint must be brought within three months starting with the date of the act complained of, subject to the same Day A and Day B extension, with conduct extending over a period treated as done at the end of that period; a tribunal may allow a further period where it is just and equitable. Take a course of treatment whose last act falls on 15 June 2026: the basic limit expires at the end of 14 September 2026. This claimant notifies Acas promptly, on 26 June 2026 – Day A – with 80 days left; conciliation runs the full twelve weeks and the certificate arrives on 18 September 2026 – Day B.
| Milestone | Date | Why |
|---|---|---|
| Basic limit | 14 September 2026 | Three months starting with the last act |
| Day A | 26 June 2026 | Acas notified with 80 days left |
| Day B | 18 September 2026 | Certificate after the full twelve weeks |
| Shifted deadline | 7 December 2026 | Basic limit plus the 84 paused days |
| One-month floor | 18 October 2026 | One month after Day B – earlier, so not engaged |
| Final deadline | 7 December 2026 | The later of the two |
The lesson is the mirror image. The earlier Day A falls, the more of the twelve weeks converts into extra time: a mid-September deadline becomes an early-December one. The one-month floor never came into play – it is a rescue device, not the main engine.
Worked example: unlawful deduction from wages
A complaint about an unlawful deduction from wages must be presented before the end of the period of three months beginning with the date of payment of the wages from which the deduction was made, again with the Day A and Day B extension. Take a short payment on the pay day of 28 August 2026: the basic limit expires at the end of 27 November 2026. This claimant hesitates – hopes payroll will fix the error informally – and notifies Acas only on 20 November 2026, Day A, with seven days left. Conciliation runs the full twelve weeks; the certificate arrives on 12 February 2027, Day B.
| Milestone | Date | Why |
|---|---|---|
| Basic limit | 27 November 2026 | Three months from the date of the underpaid wages |
| Day A | 20 November 2026 | Acas notified with seven days left |
| Day B | 12 February 2027 | Certificate after the full twelve weeks |
| Shifted deadline | 19 February 2027 | Basic limit plus the 84 paused days |
| One-month floor | 12 March 2027 | One month after Day B |
| Final deadline | 12 March 2027 | The later of the two |
This is the one-month rule at full stretch. The claimant arrived at Acas with seven days to spare and leaves with a deadline more than six months after the underpayment. The rescue works only because Day A fell inside the basic limit: a claimant who waits until 28 November 2026 gets nothing, because there is no unexpired limit left to extend.
What changes on 1 October 2026
Schedule 12 to the Employment Rights Act 2025 substitutes six months for three across the tribunal limitation provisions, including those for unfair dismissal, unlawful deductions from wages and discrimination at work; at the time of writing that Schedule is prospective. Acas guidance confirms that from 1 October 2026 the time limit for most claims becomes six months minus one day, applying where the time limit starts on or after that date. Nothing else in the machinery changes: Day A, Day B, the pause and the one-month floor work identically. Only the starting period doubles.
Run on a post-change trigger date, the same computation shows the combined effect. A dismissal with an effective date of termination of 5 October 2026 carries a basic limit expiring at the end of 4 April 2027. Notify Acas on 2 November 2026, let conciliation run twelve weeks, and Day B falls on 25 January 2027. The shifted deadline is 27 June 2027; the one-month floor, 25 February 2027, is not engaged. The final deadline is 27 June 2027 – close to nine months after the dismissal. All three worked examples above use trigger dates before 1 October 2026 and remain correct for them; claims arising on or after it run through the same steps with a six-month starting period.
The calendar discipline
The discipline the arithmetic demands is unglamorous. Compute the basic limit on the day the problem happens, and write it at the front of the file. Diarise Day A on the day Acas is notified, because the pause only protects a claimant who notifies Acas within the basic time limit, and going through a grievance, disciplinary or appeal procedure does not move the limit at all. Never budget for the full twelve weeks: the certificate controls the computation, and it can arrive in week one. When it arrives, compute both dates – the shifted deadline and the one-month floor – take the later, and diarise a working deadline a fortnight earlier still, so a rejected form cannot turn a computed date into a missed one.
The public lesson is simple. A twelve-week conciliation period is twelve weeks of settlement opportunity and twelve weeks of calendar risk at the same time. The claimant who treats every date in the chain as a computed event keeps the benefit of the first and avoids the second.
Source anchors
The framework and the worked arithmetic in this article rest on the following official sources. Source anchors support the framework described; they are a starting point for checking the position in any individual case, not a substitute for the current text.
legislation.gov.uk. Section 18A imposes the requirement to contact Acas before instituting most proceedings and bars presentation of a claim without an early conciliation certificate.
legislation.gov.uk. Section 111 (unfair dismissal) and section 23 (unlawful deductions) set the three-month limits, and section 207B supplies the Day A and Day B extension and the one-month floor.
legislation.gov.uk. Section 123 sets the three-month discrimination limit and the just-and-equitable discretion; section 140B mirrors the Day A and Day B machinery for discrimination complaints.
legislation.gov.uk. The Early Conciliation Rules in the Schedule: rule 6 sets the conciliation period – twelve weeks for notifications from 1 December 2025 – and rule 9 governs the certificate and its deemed receipt.
legislation.gov.uk. Schedule 12 increases tribunal time limits from three months to six across the main employment claims; prospective at the time of writing, with effect for time limits starting on or after 1 October 2026.
GOV.UK. The official claiming guidance: the usual three-month limit, the need for an early conciliation certificate for each respondent, and the routes for claiming online or by post.
acas.org.uk. Acas explains the pause to the time limit during early conciliation, the need to notify within the limit, and the move to six-month limits for time limits starting on or after 1 October 2026.
The closing point
The merits matter. But procedure decides when the merits are heard. Doubling the conciliation period did not make tribunal deadlines kinder or harsher; it made them more computed. The claimant who extracts Day A, Day B, the shifted deadline and the one-month floor – and diarises all four – is the claimant whose good case is still alive to be heard.
Limitation timetable decision point
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