Link to the live demo site where you can see what this code does (aside from running the actual server): https://freeciv.andrewmcgrath.info
Why 23 hours? Is this a typo?
The logic here is that we have players that are in Toronto Canada, Portland (Oregon) USA, Newcastle Australia and Berlin Germany. If we put the time at 24 hours, it would mean the turns are scheduled to end approximately the same time every day which introduces potential advantages / disadvantages to certain players.