I am now running Ubuntu 11.04, but I remember that in previous versions of Ubuntu you had an interesting functionality in the clock indicator. You could see how late it was in some places in the world different from your current one. This was associated with a world map showing where it was day and where it was not.
How can I get this functionality back in the clock indicator ? Otherwise, do you advice me another solution to know easily how late it is in some special places ?
I am also missing same functionality. However, if you are looking for current time at different places then Time & Date indicator applet could be off your help.
To show the current time at different places, follow this procedure.
Click on Time indicator applet -> Click on Time & Date Settings... -> Click on Clock tab -> Select Time in another location checkbox -> Click on Choose Locations..
Now enter the name of the place and wait for applet to search it. Once it returns the results, then select the place you want to display.
Now come out, all places you have selected will be visible in your time applet indicator along with their current time.
I too missed the display of world daylight (one reason I stuck to Ubuntu 10.04 until now. The good news is that if you update to Ubuntu12.04 and install Maté you not only get rid of Unity, but if you add clock to a panel you get the daylight display back :-)
Orage Globaltime
You could install Orage Globaltime. The shown time zones and the number of shown time zones is fully configurable. You can add its launcher to the desktop panel.