Hello,
You are able to click on the newspaper, and below Index Of Nations, click on the "game info" tab, it will tell you when the next day change is. Also, we can calculate our timezone difference from that of Germany, and know how many hours we are off, and read accordingly. So day change is around 1:00am for Germany, roughly eight hours ahead of me. So 9:00am someone joins the game, I know they joined 1:00am my time. 
Hope this helps.
