Hi Ruth,
Thanks for visiting the Disney Parks Moms Panel with your vacation planning question!
Once you physically activate your
theme park tickets at a park entrance, your 14-days begins at that time. When purchasing online, they are asking you when you
think you might first use the tickets, but you are not obligated to use them for that exact date. It is a "crowd projector" of sorts which can help them get a general idea of when people might be visiting the parks. If you decide to ring in the New Year from the comfort of your Walt Disney World Resort Hotel room, and not visit the parks until January 2nd, that is perfectly fine.
Please let us know if we can assist you further!
Wishing you magical memories,
Kaylene