Short answer. No. A marriage settlement must be executed before the celebration of the marriage, so a document signed after the ceremony is not a prenuptial agreement at all. Your property relations are governed by whatever regime the law supplied on the wedding day itself.
What the law says
The marriage settlements and any modification thereof shall be in writing, signed by the parties and executed before the celebration of the marriage.
Family Code, Article 77 — Form of Marriage Settlements; Effect on Third Persons. Read the full provision →
The timing is the whole requirement
Three things are demanded of a marriage settlement and the last is the one couples miss. It must be in writing, it must be signed by the parties, and it must be executed before the celebration of the marriage. A settlement is an agreement about how you will own property once you are married; the law lets you make it only while you are still free to walk away from the marriage entirely. Once the ceremony is over that freedom is gone, and so is the window. The same sentence applies to any modification thereof, so a couple cannot amend a genuine prenup after the wedding either.
What governs you instead
The consequence is not a gap. Where there is no valid settlement, the law itself fixes the regime that applies from the moment of celebration, and for marriages under the Family Code that default is the absolute community of property. So the couple who signed a week late are not unregulated; they are simply under the regime they were trying to avoid. This matters most to the spouse who assumed a separation-of-property arrangement was in place and continued for years to treat earnings and acquisitions as personal. The paper trail they built means nothing against the regime the law imposed.
The late document may still do something
A void marriage settlement is not automatically worthless as evidence. Depending on what it says, it can record whose money bought a particular asset, what each spouse brought into the marriage, or that a specific property was received by one of them alone. Those are questions of fact that arise later, when the community is liquidated or a creditor comes forward, and a contemporaneous signed statement is useful proof even though it cannot change the regime. Do not expect it to do more than that, and do not represent it to a bank or a buyer as a prenuptial agreement.
Where the date is actually recorded
Before concluding the agreement is late, check the documents rather than memory. The notarial acknowledgement on the settlement carries a date, the marriage certificate carries the date of celebration, and the two together settle the question in a way recollection cannot. Couples who signed drafts across several sittings sometimes find the operative signature page predates the wedding after all. If it genuinely does not, the practical question shifts to what your property relations look like under the default regime, and that is the conversation to bring to a lawyer.