The King's Calendar (KC) is a standardized time-keeping that has found commonplace use across Azeroth. Its origin is with the Alliance, hence its namesake, and its year 0 and appears to coincide with key events concerning the Council of Tirisfal and the return of the Guardian Aegwynn.
Despite this, its longevity has made it a convenient standard reckoning to use even for non-Alliance groups and factions.
An alternative to it is by dates reckoned Before and After the Dark Portal (BDP, ADP) -- which puts year 592 KC as year 0 ADP. This coincides with the first orc invasion through the Dark Portal, and puts the start of this campaign at year 25 After the Dark Portal.
Note from GM!
Easiest way to see all that has happened is to view the calendar by year, and the Reminders in the page sidebar will list all chapters that have been dated to the calendar.