Royal Observatory Greenwich
From sundials to research on time
The Royal Observatory Greenwich has a wide-ranging treasure trove of information on time & Astronomy.
British Horological Institute - packed full of materials and links
Compute Local Apparent Sidereal Time that is the actual time for your spot on the globe in relation to Greenwich. Outside the USA you will have to know your longitude (that's the East, West one!). Greenwich is at zero degrees Prime Meridian of the World.