There were several romantic relationships featured in The Haunting of Bly Manor, but Dani and Jamie's lasted the longest — even so, how long were they actually together before Dani's tragic sacrifice?
Danielle "Dani" Clayton is portrayed by Victoria Pedretti, a returning cast member from Mike Flanagan's first installment in The Haunting series, The Haunting of Hill House. After living in London for several months, she acquires a job at Bly Manor as an au pair to Miles and Flora Wingrave, who lost their parents in a tragic accident. When she first sees Jamie (Amelia Eve), the gardener, it is love at first sight. The narrator, who is actually Jamie in the present telling the story to an adult Flora on the eve of her wedding, reveals that the au pair felt as if she had known the gardener all of her life even with just a glance. After Dani allows the lady in the lake to be tucked away inside of her, the couple realizes that they have an unknown amount of time to together until Viola's spirit takes her back to Bly.
As episode 9, "The Beast In The Jungle", progresses, moments of Dani and Jamie's time spent together and the life they built with each other are shown. After the events at Bly Manor, Dani and Jamie moved to Vermont, opened up a plant and flower shop, and gained civil union status as a same-sex couple, since marriage had not yet been legalized in the United States at the time. Dani and Jamie fell in love in 1987, here's how long they were together.
In order to uncover how long Dani and Jamie were together, the history of marriage equality and same-sex civil unions needs to be examined. There is no other indication of how long time has passed since they left Bly except for their civil union. In the state of Vermont, civil unions for same-sex couples were legalized on July 1, 2000. When Jamie returns to the apartment they share together, she announces that the line was ridiculously long, but now they are officially in a civil union with one another. This means that it is likely that she went to obtain the document as soon as it happened based on how long she describes waiting for it.
Since they began their relationship in 1987, they were together for approximately 13 years if Dani returned to Bly in 2000 after they obtained their civil union status. It is possible that Mike Flanagan chose this amount of time in order to tie together Viola and her husband's time spent together, but they were only married for approximately 6 years before she died at the age of 35. It's likely the director chose 13 years in order to make Dani the same age as Viola was when she died. While the au pair's age is never specified, she's possibly in her early twenties, based on her backstory with her ex-fiancé and the amount of time that goes by between the two of them. If she is 22 in 1987, then 13 years would make her the same age as Viola when she returned to the lake in 2000.
While the exact reason for choosing the number 13 has not been specified by Mike Flanagan, it's probably based on its history as an unlucky number or even a way to tie Viola and Dani together by making them reach the same age in life before an untimely death at Bly Manor. Tragic deaths are a recurring theme in The Haunting of Bly Manor, and while Viola's death wasn't necessarily as tragic as Dani's, it created the continual hauntings at Bly, and Dani's ended the cycle — this might be another reason for Dani being potentially the same age as her in death.