This is a delightful movie. The concept, REALISTICALLY speaking, is ridiculous. Of course it is!
So trying to rationalize how Johnny could have met up with Samantha and gotten her to fall in love with him, or how Johnny and his mother could exist in a little "bubble" of the 1920s is counterproductive. Take this film without questioning it too deeply... suspend your disbelief.
If you can manage that, you will discover this to be a charming little film that is well worth watching.
The period language, dancing and music were a real treat.