The story begins on Queen Victoria's Diamond Jubilee, 20 June 1897, at which time Kathleen claims to be 5 years old; the story then progresses to 1918, making Kathleen now 26 years old. In 1941, when the production was filmed, Jeanette MacDonald claimed to be 34, but was, in fact 38, so even with careful makeup and soft focus photography, it was still a bit of a stretch.
There's only one thing missing from this picnic. Don't you have any ants in England?
Kathleen: I have some in Ireland, but I never hear from them.
The flashback to the party before John and Moonyean's wedding includes an important scene in the garden involving only Moonyean and Jeremy. The framing device of the flashback is John telling Kathleen what happened back then. However, since Moonyean died and Jeremy disappeared forever the next day, there was no way for John to know what happened in the garden scene that he apparently relates to Kathleen.