A childhood belief about speaking

When I was very young, we were leaving my Grandmother's house, and she said to me, "I'll have to come visit you at your house one day."

I had never heard the words "one day" used in this manner. As far as I was concerned, the only usage of that term was at the beginning of a story (i.e "One day Johnny was walking down the street..."). To me, "one day" always meant something in the past, and I thought she was crazy to use it to talk about something in the future.