The Barovian Calendar originated in the Prime Material world of Barovia's origin. Its year one is dated from the foundation of Barovia by ancestors of Count Strahd von Zarovich. "Barovian Calendar" is often abbreviated BC. It seems to be the commonly used calendar in the Demiplane of Dread.
The Multiversal Timeline on this site follows the assumption that 1 BC is around the Earth year AD 875. This assumption is based on extended ties to Earth's calendar through the After Crowning calendar; more information can be found at D&D Worlds Timeline Conversion.