R. v. Marshall, 2021 ONCA 344, May 21, 2021, at paragraphs 51 to 53:
It is also important to appreciate and maintain the clear distinction between the “Summers” credit and the “Duncan” credit. The “Summers” credit is a deduction from what the trial judge determines to be the appropriate sentence for the offence. The “Summers” credit is calculated to identify and deduct from the appropriate sentence the amount of the sentence the accused has effectively served by virtue of the pretrial incarceration. The “Summers” credit is statutorily capped at 1.5:1. It is wrong to think of the “Summers” credit as a mitigating factor. It would be equally wrong to deny or limit the “Summers” credit because of some aggravating factor, such as the seriousness of the offence: R. v. Colt, 2015 BCCA 190.
The “Duncan” credit is not a deduction from the otherwise appropriate sentence, but is one of the factors to be taken into account in determining the appropriate sentence. Particularly punitive pretrial incarceration conditions can be a mitigating factor to be taken into account with the other mitigating and aggravating factors in arriving at the appropriate sentence from which the “Summers” credit will be deducted. Because the “Duncan” credit is one of the mitigating factors to be taken into account, it cannot justify the imposition of a sentence which is inappropriate, having regard to all of the relevant mitigating or aggravating factors.
Often times, a specific number of days or months are given as “Duncan” credit. While this quantification is not necessarily inappropriate, it may skew the calculation of the ultimate sentence. By quantifying the “Duncan” credit, only one of presumably several relevant factors, there is a risk the “Duncan” credit will be improperly treated as a deduction from the appropriate sentence in the same way as the “Summers” credit. If treated in that way, the “Duncan” credit can take on an unwarranted significance in fixing the ultimate sentence imposed: R. v. J.B. (2004), 187 O.A.C. 307 (C.A.). Arguably, that is what happened in this case, where on the trial judge’s calculations, the “Duncan” credit devoured three-quarters of what the trial judge had deemed to be the appropriate sentence but for pretrial custody.