"Mary and Tim" is in a lot of ways, better than the 1979 movie "Tim". One thing to note, anyone who thinks Justine is hostile in this movie (or Dawnie in the 1979 version), would be surprised by the book.
Tims' family is a aware of Marys' age in this version well than before in the book or other movie. It is a cuase of tension, but not the hostility in the two prior cases.
One difference is that this version is shot in California, as opposed to Australia. Another important version is that Mary Horton is a widow, as opposed to the two prior versions, where she was never married. As well, Mary talks about her departed mother, while in the novel, she never knew her parents.
Also, in this version, Tim is more emotionally empowered. It is him who asks Mary to marry him. Not some outsider. He also is shown to have a lot of insight into what Mary wants, well before Mary is willing to admit it to herself.
After Tim proposes marriage, before Mary responds, his father asks if he and Mary have had sex. Tim is shown as knowing about sex, although I will not say if they do have sex before marriage.
It is a very good version. I always enjoy seeing it.