Having attributes for month leaving and returning sounds like a really good idea. Let me know if you guys want help with that query or function.
We only have a few snowbirds, and we ask them to include us on their list of things to do when they come and go, and then change their primary address when we hear from them. We do also have a check-box attribute for snow birds, however it's primary purpose is to have the MIA report ignore them during the winter months.