Massachusetts law permits the Commonwealth to file a petition for civil commitment of an individual as a sexually dangerous person (SDP) only if that individual is a lawful prisoner at the time of the presentation of the petition to the Superior Court. G. L. c. 123A, § 12(b).1 Coffin v. Superinten*627dent, Mass. Treatment Center, 458 Mass. 186, 188-189 (2010). “SDP commitment is only available if the procedure is initiated before the termination of a period of criminal confinement; if it is not, the statute no longer applies.” Commonwealth v. Gillis, 448 Mass. 354, 359 (2007). However, “[t]he fact that a person is in State custody is not enough to make him a ‘prisoner’ within the meaning of § 12(h).” Commonwealth v. Allen, 73 Mass. App. Ct. 862, 864 (2009). The Commonwealth may not file an SDP petition against prisoners who are in custody “solely as a result of a clerical error,” Commonwealth v. Allen, supra, or “incarcerated under a facially unconstitutional statute.” Coffin v. Superintendent, Mass. Treatment Center, 458 Mass. at 189.

After a civil trial, a Superior Court judge found the defendant to be an SDP as defined in G. L. c. 123A, § l, 2 and committed him to the Bridgewater Treatment Center (center) for one day to life pursuant to G. L. c. 123A, § 14(d). On appeal, the defendant argues that the judge should have dismissed the petition because he was not a lawful prisoner at the time of the filing of the petition on December 17, 2007. To resolve that argument we must work our way through a chronology of offenses and through multiple concepts and measurements of the law of criminal sentencing. For the following reasons, we affirm the judgment establishing the defendant’s SDP status and commitment.

Background. 1. Defendant’s criminal history. In May of 1995, the defendant pleaded guilty in Superior Court in Barnstable County to a single count of indecent assault and battery on a child under age fourteen, G. L. c. 265, § 13B. The indictment charged him with commission of the offense in Eastham during August of 1993. Also in May of 1995, the defendant pleaded *628guilty in Rhode Island Superior Court to three counts of second degree child molestation. See R.I. Gen. Laws § 11-37-8.3 (2006). The Rhode Island charges were that he had assaulted the same Massachusetts juvenile and two other boys in Newport during 1993. On the Massachusetts offense he received a suspended sentence of from nine to ten years; and on the Rhode Island offense, a suspended sentence of ten years. Both sentences included probationary terms; the Massachusetts sentence was to run concurrently with the Rhode Island sentence.

In 1997, the defendant moved to California. Rhode Island and Massachusetts transferred his probation supervision to that State. In January of 1998, police arrested him for providing marijuana to a minor. He pleaded guilty to contributing to the delinquency of a minor, Cal. Penal Code § 272 (2008). In June of 1998, extradition proceedings returned him to Rhode Island. In September, a Rhode Island judge re-probated him with additional conditions. In November, police in Rhode Island arrested him on a new charge of child molestation. In June, 1999, a Rhode Island judge found him in violation of probation and imposed the original sentence of ten years in State prison. Meanwhile, the Massachusetts Department of Probation lodged a warrant with the Rhode Island Department of Correction for the subsequent appearance of the defendant in Superior Court in Barnstable County for revocation of probation and imposition of the Massachusetts sentence.

2. Defendant’s incarceration. The defendant remained imprisoned in Rhode Island until his parole in May of 2004. During incarceration, he participated in multiple rehabilitative programs and as a result earned 570 days of good time credit. Upon his parole, Massachusetts took custody of him. The Department of Probation charged him with violation of the terms upon which his suspended Massachusetts sentence depended. He remained confined in a house of correction for the ensuing two years while his counsel sought to dismiss the Massachusetts probation surrender proceeding and to resolve an open charge in Rhode Island.

In June, 2006, a Superior Court judge conducted a surrender hearing and found the defendant in violation. On August 18, 2006, the judge imposed the original sentence of from nine to *629ten years. The judge credited the defendant with 3,028 days (or eight years and 106 days) for time served in Rhode Island and Massachusetts.3 The judge did not grant any credit for the good time earned in Rhode Island. On August 23, 2007, the Massachusetts Department of Correction (DOC) notified the Barn-stable district attorney of the defendant’s anticipated release date of May 3, 2008.

3. Defendant’s motion to correct the mittimus. On October 22, 2007, the defendant filed a motion in Superior Court to correct the mittimus4 so that his Massachusetts sentence would run nunc pro tunc from November 4, 1998, the date of his initial detention for violation of his Rhode Island probation, rather than August 18, 2006, the date of his probation surrender hearing and sentencing in Massachusetts.5 The defendant sought this change so that he could accrue earned good time in Massachusetts, pursuant to G. L. c. 127, § 129D, for his participation in rehabilitative programs during incarceration in Rhode Island. That provision confers discretionary authority upon *630DOC to grant such credit.6 Because the mittimus showed that the defendant started his Massachusetts sentence on August 18, 2006, DOC notified him that he lacked the status of a sentenced prisoner for purposes of G. L. c. 127, § 129D, while he was incarcerated in Rhode Island and that therefore he could not receive Massachusetts earned good time during that period. See McNeil v. Commissioner of Correction, 417 Mass. 818, 826 (1994) (only sentenced prisoners may receive earned good time under § 129D). DOC advised the defendant that he would be eligible to receive earned good time for his participation in the Rhode Island programs if a corrected mittimus designated November 4, 1998, as the starting date of his sentence.

On December 6, 2007, the judge allowed the defendant’s motion to correct the mittimus; however, the judge wrote in the margin of the motion that “the effective date of this allowance will be January 2, 2008.” On January 2, 2008, the judge reconsidered his decision on the defendant’s motion to correct the mittimus and wrote, “Upon further review, the defendant’s release date on this matter shall be April 12, 2008.”

By letter sent to the court on March 13, 2008, DOC advised the judge that his two orders on the defendant’s motion to correct the mittimus conflicted “because although the December 6 endorsement might result in a discharge earlier than March 29, 2008, the January 2 endorsement provides that the release date from the sentence ‘shall be April 12, 2008.’ ” The judge then issued a formal order on April 16, 2008, explicitly designating the starting date of the defendant’s sentence as November 4, 1998.7

4. Defendant’s motion to dismiss the SDPpetition. Meanwhile, on December 17, 2007, the Barnstable district attorney filed the *631petition pursuant to G. L. c. 123A, § 12, for civil commitment of the defendant as an SDR The defendant moved to dismiss the SDR petition as untimely. He argued that he was entitled to the earned good time for his participation in the Rhode Island programs and that such credit would have brought his sentence to expiration before the date of the Commonwealth’s SDR petition.

In May of 2008, the judge conducted a hearing upon the motion. Sentencing counsel for DOC testified that DOC could not award the Rhode Island good time earned before the Massachusetts sentencing in August, 2006, because until then, the defendant occupied the status of a pretrial detainee ineligible for credit under the rule of McNeil v. Commissioner of Correction, 417 Mass. at 826. Sentencing counsel testified further that the judge’s April 16, 2008, order correcting the mittimus made the defendant eligible to receive earned good time for the Rhode Island programs but that DOC had not evaluated the merits of his request because his criminal sentence had already expired.

The judge concluded that the defendant had remained a lawful prisoner as of the petition date of December 17, 2007, and denied the motion to dismiss. The SDR process went forward to trial, adjudication of SDR status, and commitment to the center.

Analysis. The defendant contends that the judge effectively corrected the mittimus of his Massachusetts sentence on December 6, 2007, so as to establish its starting date as November 4, 1998; and that the correction per se credited him with the 570 days of Rhode Island earned good time so as to bring his Massachusetts sentence to completion before the petition date of December 17, 2007,8 and to entitle him to dismissal of the petition. The following reasons defeat that argument.

First, the judge’s allowance on December 6, 2007, of the motion to correct the mittimus included the reservation that the *632effective date of the allowance was to be January 2, 2008, and, on the latter date, he reconsidered the motion and corrected the release date to April 12, 2008. Thus the actual date on which the mittimus was corrected to give the November 4, 1998, start date was not December 6, 2007, but rather April 16, 2008, or some four months after the day the Commonwealth filed the SDP petition. See note 7, supra, and the accompanying text. The judge’s expression on December 6, 2007, of a willingness to allow the correction on January 2, 2008, was not a presently effective ruling or order. “A judge’s power to reconsider his own decisions during the pendency of a case is firmly rooted in the common law, and the adoption of [a rule of criminal procedure] was not intended to disturb this authority.” Commonwealth v. Haskell, 438 Mass. 790, 792 (2003). See Commonwealth v. Downs, 31 Mass. App. Ct. 467, 469 (1991) (“Judges are not condemned to abstain from entertaining second thoughts that may be better ones”). He retained discretion on January 2 to defer action to April.

Second, even if December 6, 2007, were the effective date of the allowance of the motion to correct the mittimus, the action would not have accomplished the accreditation of the desired 570 days. The motion called for the judge to amend the starting date of the Massachusetts sentence; it did not request the award of the Rhode Island earned good time. The mittimus correction simply made the defendant a sentenced prisoner from November 4, 1998, onward, and thereby rendered him eligible to receive the Rhode Island good time credit.9 In accordance with G. L. c. 127, § 129D, discretion to award earned good time to prisoners remained vested in the DOC. See note 6, supra; Piggott v. Commissioner of Correction, 40 Mass. App. Ct. 678, 684 (1996) (§ 129D “unambiguously gives the commissioner discretion whether or not to award earned good time credits for participation in rehabilitative programs”). A prisoner does not have any entitlement to earned good time until the commissioner acts.10 *633See Haverty v. Commissioner of Correction, 440 Mass. 1, 5-6 (2003) (prisoners have no statutory or constitutional right to earned good time). See also Jackson v. Hogan, 388 Mass. 376, 379 (1983). Therefore, to receive earned good time for his participation in the Rhode Island programs, the defendant needed still to petition the commissioner and to receive a favorable decision. Furthermore, that credit must have accrued to him before the petition date of December 17, 2007. No statutory or regulatory source provides for retroactive effect for good time credit in the present circumstances.

Finally, the exercise of independent judgment by DOC would operate with special importance in these circumstances. The case required DOC to evaluate the quality of rehabilitative programs administered not under its own auspices, but rather those of a sister State. We cannot assume that DOC would mechanically adopt the determination by a separate government of the quality of rehabilitation or the quantity of credit attributable to those programs.11 In the proper exercise of the police power, *634DOC retained separate sovereign authority and nondelegable responsibility to assess the entitlement and the timing of the release of a recidivist offender into its own population.12 In this instance, it never received the occasion necessary for the exercise of that judgment.

Conclusion. As of the date of the Commonwealth’s submission of a petition under G. L. c. 123A, § 12(¿>), DOC had not exercised its discretionary judgment of his entitlement to sentence credit. That determination was necessary for an entitlement to release before the petition date. He therefore remained a lawful prisoner subject to the process of the SDP statutory scheme.

Judgment and order of commitment affirmed.