Timeanddate.com – A world clock, calendar and more.
I sometimes have to work with people in other time zones and I often use timeanddate.com to help me get my head around time differences between different regions. There are other things you can do there as well. You can get a localised calendar showing moon phases as well as local holidays and observations. You can create your own calendars, calculate dates, find times for sunrise and sunset for pretty much any city in the world, research Equinox and Solstices, find out the next eclipse, view time based maps and learn more about new and different dates of significance (eg. April 8 2013 is Yom HaShoah in Australia where the Jewish community remembers the Jewish people who dies during the Holocaust between 1933 and 1945.)
The site also has links to its own apps for Android, Windows 8 the iphone and ipad.
The site is free to use and also free to register which allows you to save your favourite settings.