USPS does not have on official listing of Time Zones by Zip Code. They have up to date street names, 2006/7 lat/lng and up to date city/states.
Also, AT&T (phone) does not have any time zone data from phone area codes (at least they said they don’t)
I am looking for the most accurate way to get time zones by either zip code, city or phone number. Whatever is more likely to be accurate. I can make sure the results are not drastically off by looking at the state., but that could still be an hour off.
The focus here is accuracy. What is the most reliable way and/or most trusted source(s). Any help would be appreciated. If it costs something, then that is OK. Don’t withhold the suggestion, especially if you think you can speak to its accuracy.
The concern about Phone Number is that some people use out of state area codes from VoIP services. But if that is the most up to date way, we might just use it anyway.
Started with states, had a co-worker come up with the zip codes that were not in the biggest TZ for their state. I don’t know how he did it but it was a manual process.
For fun, look up Time In Indiana on Wikipedia. It is a joke-looking history.
GeoNames looks good for an API. Get lat/lng from zip and then TZ