Yes, every day from the arrest date counts. The 163 days he spent in custody between his arrest on September 18, 2020 and his plea on February 28, 2021 are all credited toward the sentence, day for day. The clock starts at booking, not at sentencing.
Here is how the math works. Five years is 60 months. With 15 percent good time applied, he serves approximately 51 months total rather than the full 60. Subtract the roughly 5.4 months he already served pre-plea, and the remaining time from the plea date forward is approximately 45 to 46 months.
Working from February 2021, that puts the estimated release somewhere around late November 2024 to early 2025, which lines up with the 2025 release date showing in the system. The system date may reflect the full sentence end without all credits fully applied yet, or it may include a modest buffer.
If he remains clean on conduct, that 2025 date should hold or potentially come in slightly earlier. Picking up a disciplinary infraction can cost good time and push it back, so staying out of trouble is the most direct thing he controls between now and release.