The Haunting of Bly Manor is as follows: In 1984, when Flora is 5 and Miles is 7, Owen is hired and Flora sees her first ghost.
Two years later the parents die. Rebecca Jessel comes to look after the orphans and has an affair with Peter Quint. By the time 1987 rolls around, Peter is dead (though everyone thinks he just disappeared), Rebecca is dead, and the children are being possessed by the former lovers.
Hannah dies just after Dani is hired to be Rebecca's replacement. More lore reveals itself later, but the majority of the story takes place in this time frame.
Hannah is not the only one at Bly Manor who drifts off and has trouble knowing when and where she is. It happens to the children as well.