What does Tom encounter on his nightly visits to the magical garden in the novel, Tom's Midnight Garden?

Tom's nightly excursions into the garden take him into a magical world where the past and present intersect. H finds that h e can visit with a girl from the past (Hatty), who becomes his friend. For Tom the summer passes in days, but it seems to pass in years for the little girl, who gradually grows up and matures with each visit.