Sundial is a telephone-number to timezone conversion tool. Give it a US telephone number (in any old format) and it'll tell you the timezone to which the number corresponds. It'll even print out a quick table showing how the timezone corresponds to GMT.
Simply enter a US telephone number that you would like to lookup. Format is not important.
There are other ways to use Sundial besides visiting this page and typing a number into the box. One way to use Sundial is to visit a URL like http://benjaminasmith.com/tools/sundial/somenumber. Just replace somenumber with the number you want to convert. For example, to convert Mobile: 651.342.2323 you would visit the following URL.
http://benjaminasmith.com/tools/sundial/Mobile:%20651.342.2323
My favourite way to use Sundial is via Firefox Smart Keywords. Using smart keywords, I can just type:
sun Mobile: 651.342.2323
To set this up, just create a new bookmark to the following URL and give it the keyword sun.