Tex. Code of Criminal Procedure § 42A.559

(a) For purposes of this article, "diligent participation" includes:

(1) successful completion of an educational, vocational, or treatment program;

(2) progress toward successful completion of an educational, vocational, or treatment program that was interrupted by illness, injury, or another circumstance outside the control of the defendant; and

(3) active involvement in a work program.

(b) A defendant confined in a state jail felony facility does not earn good conduct time for time served in the facility but may be awarded diligent participation credit in accordance with Subsection (f) or (g).

(c) A judge:

(1) may credit against any time a defendant is required to serve in a state jail felony facility time served in a county jail from the time of the defendant's arrest and confinement until sentencing by the trial court; and

(2) shall credit against any time a defendant is required to serve in a state jail felony facility time served before sentencing in a substance abuse felony punishment facility operated by the Texas Department of Criminal Justice under Section 493.009, Government Code, or other court-ordered residential program or facility as a condition of deferred adjudication community supervision, but only if the defendant successfully completes the treatment program in that facility.

(d) A judge shall credit against any time a defendant is subsequently required to serve in a state jail felony facility after revocation of community supervision time served after sentencing:

(1) in a state jail felony facility; or

(2) in a substance abuse felony punishment facility operated by the Texas Department of Criminal Justice under Section 493.009, Government Code, or other court-ordered residential program or facility if the defendant successfully completes the treatment program in that facility.

(e) For a defendant who has participated in an educational, vocational, treatment, or work program while confined in a state jail felony facility, the Texas Department of Criminal Justice shall record the number of days during which the defendant diligently participated in any educational, vocational, treatment, or work program.

(f) For a defendant with a judgment that contains a finding under Article 42.0199 that the defendant is presumptively entitled to diligent participation credit and who has not been the subject of disciplinary action while confined in the state jail felony facility, the department shall credit against any time the defendant is required to serve in a state jail felony facility additional time for each day the defendant actually served in the facility while diligently participating in an educational, vocational, treatment, or work program.

(g) For a defendant with a judgment that contains a finding under Article 42.0199 that the defendant is not presumptively entitled to diligent participation credit or who has been the subject of disciplinary action while confined in the state jail felony facility, the department shall, not later than the 30th day before the date on which the defendant will have served 80 percent of the defendant's sentence, report to the sentencing court the record of the number of days under Subsection (e). The contents of a report submitted under this subsection are not subject to challenge by a defendant. A judge, based on the report, may credit against any time a defendant is required to serve in a state jail felony facility additional time for each day the defendant actually served in the facility while diligently participating in an educational, vocational, treatment, or work program.

(h) A time credit under Subsection (f) or (g) may not exceed one-fifth of the amount of time the defendant is originally required to serve in the facility. A defendant may not be awarded a time credit under Subsection (f) or (g) for any period during which the defendant is subject to disciplinary status. A time credit under Subsection (f) or (g) is a privilege and not a right.