Short answer. No — the guaranty is extinguished. Article 2079 provides that an extension granted to the debtor by the creditor without the guarantor's consent extinguishes the guaranty. A binding extension of the maturity, agreed behind your back, releases you entirely. It changes the risk you undertook, so the law lets you off rather than hold you to a different bargain.
What the law says
An extension granted to the debtor by the creditor without the consent of the guarantor extinguishes the guaranty
Civil Code, Article 2079 — An Extension Given Without the Guarantor's Consent. Read the full provision →
An unconsented extension wipes out the guaranty
A guarantor undertakes a specific risk on specific terms, and maturity is one of them. Article 2079 protects that: An extension granted to the debtor by the creditor without the consent of the guarantor extinguishes the guaranty. If the creditor and the debtor agree to move the deadline and never obtained your agreement, you are not merely partly relieved — the guaranty is gone. This is a full release, not a defence to be measured against damage. The moment a binding extension is granted without your consent, the obligation you guaranteed is no longer one you are bound to answer for.
Why the law releases you
The reason lies in what an extension takes from the guarantor. On the original maturity, a guarantor who feared the debtor's slide could pay the debt and immediately turn on the debtor to recover, while the debtor still had something to seize. An extension freezes him: he cannot force the issue during the added time, and the debtor's position may worsen meanwhile. Having his hands tied by a bargain he never agreed to, the guarantor is exposed to a risk different from the one he accepted. Rather than rewrite his undertaking for him, the law simply discharges it.
It has to be a real extension
What releases you is a genuine, binding grant of new time — an agreement that the creditor cannot demand payment until a later date. It is the alteration of the term, made without your consent, that does the work. Not every act of patience qualifies: a creditor who merely holds off, tolerates a late payment, or fails to press the debt has not necessarily granted an extension in this sense. The distinction between an agreed postponement and mere forbearance is the whole battleground of these disputes, and it is taken up directly in the related question on failure to demand payment.
Consent is what keeps you bound
The release turns entirely on the absence of your consent, so consent is also the cure. If you agreed to the extension — whether at the time it was granted or in advance, through wording in the guaranty by which you accepted extensions the creditor might later give — you remain bound, because the risk was one you took on with open eyes. This is why the guaranty document matters so much. Before assuming an extension freed you, read what you signed: a clause consenting in advance to extensions is common, and it defeats the argument.