SmartyStreets will return lat/long and time zone data for United States addresses, all in one step. You can do this via API or a list upload. Just keep in mind that time zone data can change, so you would need to update your data every once in a while to keep up with those changes (if you're planning on keeping some sort of database, etc.)
Another thing to be aware of is Daylight Savings Time observance, which SmartyStreets also indicates in its returned data. You can see a full list of our output fields here.
(Disclosure: I work at SmartyStreets)
Update - 11 Jan 2016: SmartyStreets now processes international addresses.