How did Medieval sentries in big cities knew when their shift was over and how did they keep rosters? Did they have a fair rotation like person A stood midnight to 6am every day for the last 2 times, let's give him the morning sentry now or they just kind of days screw it John will do night sentry forever
I can't exactly answer your question, but clocks of different kinds have been around since antiquity. Water clocks have been around since at least 1600 BC, and hour glasses since 350 AD. Your medieval sentry might have access to either one of those as a way to keep time. Water clocks are very easy to make.
The first clock towers began to appear in the early medieval era, and became somewhat common in large cities after 1300 AD. They probably did not have mechanical faces for the most part, but were just to announce when certain hours had passed.
It's also worth noting that our idea of city guards and sentries is likely incorrect. Except for the largest cities and castles, town guards were probably volunteers in the local militia who worked out a system of shifts among themselves.
This really depended on the situation. In general for a small town or castle there would be one or two night watchmen who would in fact stand watch nearly every night.
On naval ships watches were generally dogged. To do this the day would be broken up into 4 hour segments.
First Watch: 2000-0000
Mid Watch: 0000-0400
Morning Watch: 0400-0800
Forenoon Watch: 0800-1200
Afternoon Watch: 1200-1600
Watch carefully now here's the tricky part.
First dog watch: 1600-1800
Second dog watch: 1800-2000
This system worked great if you had 2 or 3 watch sections as the dog watches would rotate the shifts back by one shift every day. They also allowed the shifts to eat dinner at a reasonable time. To keep track of time you would burn time candles or use an hourglass. 4 hours was a standard length for each candle. Every half hour the watch would ring the bell once for every 30 minutes of the watch. So 30 minutes is 1 bell 4 hours is 8 bells means time to switch the watch. Bells weren't used at night so you would send somebody to wake up the next watch.
If your ship was somehow able to have enough men to have 4 watch sections the problem solved itself as 6/4 would leave the men switching from night to day every other day.
This method of rotating the watch was also commonly used in port towns. I can't speak to whether more inland towns used it. I bet /u/jschooltiger could tell us exactly when this system started. Somebody track him down.