I often get confused on the time difference since I receive clients outside the country. This also happen most especially when there’s maintenance announcement on FFXIV:ARR.
I already looked at so many sites that does conversions and most of them are also confusing to use. Then, I found this site everytimezone.com by @amyhoy (thanks to my husband). The layout of the site is nice and simple, except for those in your face advertisements. But, as for the purpose of the site, it is very easy to use and see the difference between time-zones.
You can hold and drag the indicator with your mouse to see what time you need to know when. It also helps to know if you’re calling your friends/clients at the middle of the night in their time-zone.