Here’s an example of the London clock in Vista:
And here’s the hover clock in Windows 7:
Adding these additional clocks is extremely easy.
First right-click on the clock in the system tray, and choose Adjust Date/Time:
Click the Additional Clocks tab, and select the options and time zone as shown:
You can enter a friendly display name. I entered “London”, but you could put something relevant to you, like the name of a family member living elsewhere.