Short answer. From the day you started working. Article 281 measures probationary employment as not exceeding six months from the date the employee started working, not from the date of the contract. A contract signed weeks later, or backdated, does not move your regularisation date.
What the law says
Probationary employment shall not exceed six (6) months from the date the employee started working, unless it is covered by an apprenticeship agreement stipulating a longer period.
Labor Code, Article 281 — Probationary Employment. Read the full provision →
The statute picks the factual date
The Code could have said from the date of engagement, or from the commencement of the contract. It says from the date the employee started working, and the choice of words is not accidental. Probation exists so the employer can observe you doing the job, which means the period sensibly begins when you begin doing it. The consequence is that the paperwork cannot control the clock. Where the two disagree — you started on the fifth and signed on the twenty-third — the day you began working is the one the article measures from.
Why the gap opens up in the first place
Very few employers set out to manipulate this; the gap usually just happens. You are asked to come in while the offer is being processed, you spend a fortnight in training that nobody counts, your first weeks are run through an agency payroll, or the contract sits unsigned on a desk. Each of those quietly pushes the documentary start date later while you have in fact been working. It is worth noticing, because six weeks of slippage is the difference between a dismissal in month six being available to the employer and being far too late.
One exception, and extensions are not it
The article admits a single qualification: an engagement covered by an apprenticeship agreement stipulating a longer period. That is a specific arrangement, not a label an employer can attach to an ordinary hire. What the provision does not contain is any general power to extend probation by agreement, so a second probationary contract, a three-month extension to prove yourself, or a restart on a slightly different job title does not reset the six months. Signing such a document does not undo a status the law has already conferred on you.
How to establish your actual start date
You want a record made at the time by someone other than yourself. The first payslip, the timekeeping or biometric log for your first week, the date your identification or system account was issued, the first roster you appear on, or an email welcoming you on your first day will all serve. Keep the contract too, precisely because the date on it may be the point in dispute. If you were engaged through an agency before being taken on directly, keep those payslips as well, and take them to a lawyer together.