When I learned to know the person who is now my wife, we lived in different time zones. She was living in Seoul, Korea and I was living in Hamburg, Germany. To quickly be able to see what time it is at her home, I developed SecondClock.
SecondClock provides a Menubar item with a second Clock for a different time zone. You can chose the time zone by selecting one of the mayor cities of the world, rename it to customise the name displayed to the clock and see the history of your last selected zones.
SecondClock is available for a small price at the Mac App Store.