The Tsardom of Bulgaria, also referred to as the Third Bulgarian Tsardom, sometimes translated in English as the "Kingdom of Bulgaria", or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October 1908, when the Bulgarian state was raised from a principality to a tsardom.

The adoption of the Gregorian Calendar was an event in the early modern history of most cultures and societies, marking a change from their traditional dating system to the modern dating system – the Gregorian calendar – that is widely used around the world today. Some states adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between. A few still have not, but except for these, the Gregorian calendar is now the world's civil calendar universally, although in many places an old style calendar remains used in religious or traditional contexts. During – and for some time after – the change between systems, it has been common to use the terms "Old Style" and "New Style" when giving dates, to indicate which calendar was used to reckon them.