Confused about the timelines in the 1980 film "The Shining" ?

In the beginning, Jack was told by his future boss that a caretaker murdered his own family, then killed himself, in the hotel in 1970. He takes the job anyway and ends up in a 1920s bar scene in the hotel, where the other caretaker, Delbert Grady, tells Jack that he (Jack) was the caretaker all along. At the end, Jack dies, and his picture appears in a photo dated July 4th, 1921. So what does 1921 have to do with 1970, and in which of those years did Delbert Grady kill his own family and himself? Why did Jack appear in the 1921 photo? Just wondering if anyone can answer those questions? And don't tell me to read the book, that's not what I'm asking.