This looks great, and I love the slider. Maybe I'm just not seeing the option, but I need to be able to add and remove time zones. A lot of the confusion of these tools is having to ignore the many cities that don't concern you.
Saying the weekday on each would be great too (ex: Friday, March 21). When you're working with Australia it can be hard to remember that they're almost a full day ahead.
http://www.worldtimebuddy.com/ offers exactly this. It allows you to search for cities/timezones, shows weekdays, native time format (am/pm or 24hour depending on the timezone) and you can easily select a specific date for the comparison
Saying the weekday on each would be great too (ex: Friday, March 21). When you're working with Australia it can be hard to remember that they're almost a full day ahead.