This was very accurate and precise and, in places where there were exceptions, allowed for him to predict where new elements would be placed and their relative atomic mass, to a great degree of accuracy (jan 1, 1869 – 23h 59min, dec 31, 1869 y)
Added to timeline:
Date:
jan 1, 1869
23h 59min, dec 31, 1869 y
~ 12 months