As seen on xkcd
It arbitrarily counts from the start of the year you give it in UTC. You can also change the coordinates of where the sunrise and sunset API is checking.
I have found that the API has some issues, such as returning 1970-01-01 when there is no sunrise or sunset, no matter if the sun is up. It also does not work for 1970-01-01, so any 1970 date won't work in the program.
This might be outdated, I will be updating mainly on Replit https://replit.com/@AJLee/xkcdDaylightCalendar