What are the two most commonly used notation to denote the dates of historical events?

Chronology (from Latin chronologia, from Ancient Greek χρόνος, chrónos, "time"; and -λογία, -logia)[2] is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events

in all honesty, it really hasn't. except that hitler killed himself.