Quick answer
Calculate the end date by adding calendar months to the start date, then set the review date deliberately earlier — the end date is a deadline, not a plan.
Calendar months, not thirty-day blocks
A six-month probation starting on 15 March ends on 15 September — the same day number, six months on. Treating it as 180 days gives 11 September, four days early. Over a twelve-month probation the drift grows to about five days.
Calendar-month arithmetic keeps the day number and moves the month, which is what employment contracts mean. Where the day number does not exist in the target month, the standard treatment is to clamp to the last day: a probation starting 31 August ends 28 or 29 February.
Use this guide with the right tool: Open the Probation Period End Calculator for the calendar end date of a probation period expressed in months. If the question shifts, compare it with the Add Months to Date Calculator or the Long Service Leave Calculator.
The end date is not the review date
The most common operational failure is treating the probation end as the moment to start thinking about it. By then there is no time to gather feedback, hold a conversation, or extend the period if the contract allows — and in many jurisdictions a probation that lapses without action converts automatically to full terms.
Set the review date at a fixed offset before the end — two to four weeks is typical — and diarise it at the same time you record the end date. The offset should be long enough to accommodate an extension decision and any notice that decision requires.
Extensions have their own rules
Where a contract permits extension, it usually specifies a maximum length, a required form of notification and a deadline for giving it. An extension communicated after the original end date has passed is frequently invalid.
If an extension is agreed, recalculate the new end date from the original end rather than from the date of the conversation, unless the contract says otherwise. Then set a new review date at the same offset before the new end.
Probation and statutory rights
Probation is a contractual arrangement, not a suspension of employment law. Notice requirements, discrimination protections and many statutory entitlements apply from day one in most jurisdictions, although some rights accrue with service length.
Because the specifics vary considerably by country and employment type, use the calculator for the date and take the rest to HR or an employment law source. A date is arithmetic; what may be done on that date is not.
Worked example
A six-month probation starting 15 March 2026 ends on 15 September 2026, and a review three weeks earlier on 25 August leaves time to act before that deadline. Put your own start date through the same count, then set the review well ahead of the end date.
Month-end starts and the extension trap
The clamp described above is where probation arithmetic actually goes wrong, because month-end start dates are common — people start on the first of a month, and the first is often the 31st's neighbour in the paperwork. Work the hardest common case in full. A six-month probation beginning 31 August 2026 has no 31 February to land on, so it ends on the last day of that month: Sunday 28 February 2027. The span is 181 days. Note that the day-count shortcut fails twice here: six times thirty is 180, which lands on 27 February — a day early even before the clamp is considered.
The same shape repeats across the calendar. A three-month probation from 31 January 2026 ends on 30 April 2026. A four-month probation from 31 October 2026 ends on 28 February 2027. And the leap-year version runs in the other direction: a twelve-month probation beginning on 29 February 2028 ends on 28 February 2029, because the anniversary date does not exist in the following year. In each case the rule is the one already stated — keep the day number, clamp when the month is too short — but the practical consequence is that two probations of identical nominal length, started a few weeks apart, can end on dates whose relationship is not obvious from the start dates alone.
The extension case is where the clamp stops being a curiosity. Suppose the 31 August 2026 probation is extended by three months. Recalculated from the original start, as the section above advises, nine months from 31 August ends on Monday 31 May 2027 — the day number survives, because the measurement never passed through February. But add three months to the clamped end date instead, and 28 February gives Friday 28 May 2027: three days early, silently. The clamp is not reversible. Once a calculation has landed on 28 February, the 31st is gone from the chain, and every subsequent addition inherits the loss — six months on from 28 February is 28 August, not the 31st the employee originally started on.
The review date inherits all of this. Three weeks before the clamped end of 28 February 2027 falls on 7 February 2027, and both dates happen to be Sundays. That is not an arithmetic problem — the deadline is the deadline — but it is an operational one: a review conversation that must happen before a Sunday deadline in practice happens by the Friday, which shortens the effective offset from three weeks to two weeks and two working days. For month-end starts, then, the full sequence is worth writing down at the outset: the start, the clamped end, the extension rule measured from the original start rather than the clamped end, and a review date that respects the weekdays as well as the calendar.
Common mistakes to avoid
- Calculating probation in days. Contracts use calendar months. Day-based arithmetic drifts and produces the wrong date.
- Diarising only the end date. By the end date it is too late to act. Set the review date at the same time.
- Assuming an extension can be agreed after the end date. Most contracts require notification before expiry. Check the wording and the deadline.
Use the right calculator
Probation Period End Calculator gives the calendar end date of a probation period in months, Add Months to Date Calculator shifts a date forward by complete months, and Long Service Leave Calculator checks continuous service for entitlements.
Frequently asked questions
What if the start day does not exist in the end month?
Clamp to the last day of that month. A 31 August start gives a 28 or 29 February end for a six-month period.
Does probation change notice entitlement?
Sometimes contractually, rarely statutorily. Check both the contract and the applicable statutory minimum.
How early should the review be?
Long enough to accommodate an extension decision and any required notice. Two to four weeks is common practice.