Its wrong. Mitsuri's birthday is wrong, it was stated to be May 28th in episode 8 of swordsmith villiage, and Muzan is about 1,019 - 1,020, years old. He was taking the medicine and the doctor told him if he didnt take it he wouldnt live to see the age of 20. Muzan said that was 1,000 years ago, shown in the final episode of Swordsmith Villiage.
