It's sensitive enough that when their human leaves for work at 8am, the human scent is strong and fresh, but it fades over the course of the day. A dog knows how strong the residual human smell is at 5pm, when the human usually returns from work, so can build a sense of routine from that. Especially cool though is that they can smell the difference between 4pm and 6pm, so this is why dogs are usually super excited when their humans return from routine jobs, it's not just that they hear their friend approaching, they are functionally clock watching. They know when we're running late.)

[Citation needed]

Edit: further reading added: non-academic article https://www.npr.org/sections/goatsandsoda/2022/12/22/1139781319/can-dogs-smell-time-just-ask-donut-the-dog#:~:text=%22So%20newer%2C%20more%20recent%20odors,is%20inextricably%20woven%20into%20space.