Although Lucy and Albert meet in early 1973, Albert says that Casey was born the following winter during December 11, 1975. The following winter would have started on c. December 20, 1973.
Casey argues with Lucy that she is 9 years old, when Lucy says that she is 8. Although the movie is trying to show how the parents neglect to remember her birthday, given that her date of birth is December 11, 1975, the Casey character would still be 8 at the time the movie was released.