Most of the world uses the 'Gregorian Calendar' implemented by Pope Gregory XIII in 1582. Before that the 'Roman' or Julian Calendar was used (named after Julius Caesar).

世界上大部分地区都使用教皇格里高利十三世于 1582 年实施的“公历” 。 在此之前，使用的是“罗马” 或儒略历（以凯撒大帝的名字命名）。

So in the 16th Century, Pope Gregory XIII tweak the system again so that leap ears would occur in any year that's divisible by four, but only in centuries that are evenly divided by 400, confusing, I know.

所以在16世纪，教皇格里高利十三世再次修改了系统，使得闰年发生在每四年一次的年份中，但只有能被400整除的年份才是闰年，令人困惑，我知道。