Diane,
That's a great start.
I can't take credit for the basic idea though. Back in the mid to late 80's there used to be a PIM/Business Directory called Hotline. This DOS program (which was distributed on 5.25" floppies) would tell you your contact's local time based on their Area Code. Of course, this was before cell phones, so number portability wasn't an issue back then.
If someone (like you) could write an add-in using ZIP code data instead, I'm sure a bunch of people, including myself, would purchase it.