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
Substitute your own dates and follow the instructions that actually govern your situation. Where a result sits close to a cutoff, verify the underlying date and rule independently before relying on it.
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
Each of these runs in the browser, states its assumptions, and links to the neighbouring calculation when the question turns out to be a different one.
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.