They refer to time as shifts as well, presumably since work is so crucial. So it seems to imply the shifts are a regular length of time, so a "day" for a worker

They refer to time as shifts as well, presumably since work is so crucial. So it seems to imply the shifts are a regular length of time, so a "day" for a worker would be from the beginning of their shift to the beginning of their next shift. It's a little less clear with the Earth/Mars military. It's easy to use 8 hours for a shift like we do (and 8 hours for sleep), but our shifts are based around day/night, sleep cycles, and our human limits for work within Earth's gravity. That could change on Mars though. Since there won't be weather seasons or agriculture for a long while, work shifts will likely have a bigger influence on the decision rather than the reasons we have them on Earth.