Adding Time Zone as a Field for Account and/or Cases Module

Hey everyone!!

We have a customer base that is spread out all across the US, but we also have our own offices on each side of the country as well.  We often get requests from customers to callback at specific times and often we have to refer back to the Account to see where they are and then calculate our times.  

I have looked for a field that I could use that would generate time zone based off of address and I am coming short.  Does anyone have any suggestions that would allow me to add the Time Zone to not only the Account module but that would also generate on the Cases module?