While not specifically the year 1821, this time period is when one of the most significant categorical discoveries of archaeology was named. Christian Thomsen, a Danish archaeologist, developed the three age system to date objects in museums. These three ages were the "Stone Age," "Bronze Age," and "Iron Age."
While not specifically the year 1821, this time period is when one of the most significant findings regarding time and dating archaeological findings was discovered. Boucher de Perthes established a much deeper sense of time than what James Usher had previously established. Perthes determined that the world was significantly older than 4004 BC and thus gave archaeology a deeper, more realistic time frame to work with.