One's normal schedule for bowel movements, often conveniently aligning with other bathroom related activities, and allowing one to poop at home in a comfortable setting, rather the office, or at a restaurant during lunchtime. For example, one might have their poopclock set to the early morning, so that they can poop, and then thoroughly clean themselves in the shower.
ex: I had Taco Bell for lunch, now my poopclock is all messed up and I have to crap in my disgusting office toilet with the ratty toilet paper.
The schedule one's body sets for its regular bowel movements. This schedule can be shifted to very inconvenient times by traveling to different timezones, or dramatic changes in diet or daily activities.
I can't sleep in on my days off because my internal poop clock is permanently set to 6:30a.m.