Although the included Time app from Apple is useful enough, Time Zones + World Clock Time Converter (free with in-app purchase) goes Apple a few steps better.
The attractive GUI features the ability to rename locations, so you can call a place "Mom and Dad". The app displays the time differential in hours for all the locations you enter, and a 'quick check' mode lets you look up a date and time anywhere in the world, converting it to your local time. I could see that being useful for business calls in different time zones.
As the day moves along, the colors of the GUI change to reflect the time of day.
Time Zones app for iOS
The app is ad supported, and when you open the app a full page ad comes up that has to be dismissed, which I judge mildly intrusive. A in-app purchase of U.S. $4.99 removes the ads, but I think that is a bit too expensive. I'm getting along with the free version, and if the developer would lower the purchase price to $0.99 I think he would get more takers.
Time Zones + World Clock Time Converter is a nicely done app. It's more useful that what Apple provides, and has a nice look and feel.
The app requires iOS 7 or later. It seemed to work fine using iOS 8 beta 5, so I don't foresee any issues going forward.
Another app with similar features is World Clock Time Zone. It's free too but has a $1.99 in-app purchase which removes the ads. The screens are a little more cluttered, so I prefer Time Zones + World Clock Time Converter for quick ease of use.