This was a BBC kids' drama circa 1976 (based on Philippa Pearce's 1958 novel) about a young lad, Tom Lang (Jeremy Rampling) who was staying in his uncle Alan and aunt Gwen's old house for the summer because his brother Peter contracts measles. Whenever he heard the clock in the house strike 13 he would go outside, in his pyjamas, and find himself transported back to the Victorian era. He meets a girl called Hatty here and they have adventures. But each night he visits her, she is of a different age, and not in chronological sequence.
It was really just a supernatural reworking of The Secret Garden or even The Lion, The Witch And The Wardrobe, with similar characters.