The clock shows both time and sun position. The red line indicates the entered time. The yellow region is when the sun is up. The orange region is when the sun is on the horizon. The blue region is night and the lighter shades of blue indicate civil, nautical and astronomical twilight. Solar noon is indicated by the white line pointed up, and solar midnight by the black line pointed down.
Modern is the ordinary clock, in the time zone of the location you entered.
Roman timekeeping. For the Romans, every day was twelve hours long, because the length of an hour changed over the course of a year. Daytime hours were longer during the summer and shorter during the winter. There were also twelve hours every night, shorter in the summer and longer in the winter. The hours were numbered, so the first hour began at dawn and the twelfth hour ended at sunset.
Post Auroram is a sort of compromise between the Roman way and the modern way. The length of an hour stays the same, but the clock is shifted so that sunrise happens at the same time every day. Sunrise is 0:00 PA, an hour later is 1:00 PA, and so on. The date boundary stays at midnight, so time between midnight and sunrise uses negative numbers: an hour before sunrise is -1:00 PA.
Both of these systems fail north of the Arctic circle and south of the Antarctic circle. It's hard to base a clock on sunrise when there is no sunrise.
I wrote more about why I made these in my blog post on Daylight Saving Time. I used SunCalc.js and tz-lookup to make these tools.