Short answer. Usually yes, but not because of the rule on computing penalties. That rule fixes when a sentence starts running. Credit for the months already spent in jail awaiting judgment comes from a separate provision on preventive imprisonment, and how much is credited depends on a written undertaking you may be asked to sign.
What the law says
If the offender shall be in prison the term of the duration of the temporary penalties shall be computed from the day on which the judgment of conviction shall have become final.
Revised Penal Code, Article 28 — Computation Of Penalties. Read the full provision →
What the computation rule actually settles
Article 28 of the Revised Penal Code answers a narrower question than most people assume. It fixes the starting point of a sentence. Where the offender is already in prison, the term of the duration of the temporary penalties shall be computed from the day on which the judgment of conviction shall have become final. Where he is not in prison, a penalty involving deprivation of liberty runs from the day that the offender is placed at the disposal of the judicial authorities for the enforcement of the penalty, and all other penalties run only from the day he begins to serve. It is about when the clock starts, not about crediting past detention.
Where the credit for detention comes from
The credit itself is granted by a different provision of the Code, dealing with preventive imprisonment — the time an accused spends in custody because bail was denied, unaffordable or not applied for. A detainee who agrees in writing to abide by the same disciplinary rules imposed on convicted prisoners is credited with the full period of that detention. One who does not give that written agreement is credited with a reduced portion of it. Republic Act No. 10592, enacted in 2013, amended these rules and the related allowances for good conduct, so the current entitlements are broader than the Code's original text suggests.
Who does not get the credit
The credit is not universal. It is withheld from recidivists and from those previously convicted twice or more of the same offence, and from an accused who, having been summoned for the execution of his sentence, failed to surrender voluntarily. There are also situations where the detention simply cannot be set against anything — where the accused is acquitted, no sentence exists to credit, and any remedy for a prolonged detention lies elsewhere. Where the penalty finally imposed is one the credit cannot be applied to, the position again differs, which is why the arithmetic is worked out against the specific penalty in the judgment.
Making sure it is actually applied
The credit is not always reflected automatically. It depends on records: the commitment order, the jail's certification of the exact dates in custody, and the written undertaking on prison discipline if you signed one. Ask your counsel to have the period of preventive imprisonment stated in the judgment itself and carried into the order committing you to serve sentence, and keep copies of everything the detention facility issues. If you believe the period served has already met or exceeded the penalty, that is something to raise with counsel promptly, since the remedy has to be sought from the court rather than from the jail.