The bottled message is dated "23.8 1959" and for the bottle to float from somewhere in the Stockholm archipelago in the Baltic Sea through the sluice gates at the Old Town in Stockholm into lake Mälaren and settle where it is found would indeed take a long time. It is probably safe to assume that the black and white present day scenes in the beginning and in the end are meant to take place about the time the movie was made, in 1965. On the other hand, it would be slightly strange for all of the characters in the movie not to have been missed for around six years, and that nobody would have known about the cottage at Ensamholmen. But who knows...