Yes, the calculation of pretrial confinement credit in the military system can be affected by concurrent civilian confinement. The general rule is that a service member is entitled to day-for-day credit against their court-martial sentence for any time they spend in pretrial confinement, whether that confinement is in a military brig or a civilian jail, as long as it is related to the military offenses.
The situation becomes complex when the soldier is confined by civilian authorities for a purely civilian offense and is also awaiting a court-martial for a separate military offense. The key question is whether the civilian confinement was, in effect, also serving the purpose of detaining them for the military. If the military has placed a “detainer” on the soldier with the civilian jail, which prevents their release even if they make bail, then the time spent in civilian confinement is generally credited towards their military sentence.
A military defense attorney will carefully track all periods of confinement. They will argue to the military judge that any time spent in a civilian jail where a military detainer was in place should be fully credited against the court-martial sentence. This ensures that the soldier is not unfairly “double-punished” by serving “dead time” in a civilian facility that is not credited towards their ultimate military sentence. The judge will make a final determination based on the specific facts and the nature of the detainer.