Long before the advent of the wristwatch, mankind marked the passage of time by the height of the sun. The sun was a reliable timepiece, especially with the aid of an apparently simple, yet mathematically complex, tool. If your watch batteries die, you can always count on the sun.
One variation on the sundial is the shepherds watch. One such watch, known as an Aquitaine sundial, was named after Eleanor of Aquitaine who, in 1152, gifted a shepherds watch to King Henry II of England. By suspending the ring of the shepherds watch from a cord and dialing in the month, a ray of sunlight would spill through a tiny hole, illuminating the hour inscribed inside the ring. Eleanor's gift allowed the King to determine the hour so he could return home in time to meet her. In return, the King had a shepherds watch made for Eleanor inlaid with diamonds and engraved with the words "Carpe Diem", "Seize the Day."
The hour indicated by a sundial or shepherds watch is known as the Local Apparent Time (LAT), since two different sundials will read different times unless they fall along the same meridian line of longitude (the lines running from pole to pole). That's true today, with or without a sundial.
Today, the planet is divided into time zones, preset agreed upon longitudinal regions within which the hour is always the same. There are 24 time zones throughout the world, each set at approximately 15 degrees longitude ( 24 times 15 equals 360 degrees, a full circle around the globe). Time zones aren't exact straight lines; instead, they tend to follow geographic and political boundaries. Moving eastward, each time zone is ahead one hour.
San Diego, California, for example, falls within Pacific Standard Time (PST). Traveling eastward, the subsequent continental United State's time zones are known as Mountain Standard (MST), Central Standard (CST), and Eastern Standard Time (EST). Therefore, when it's 10:00am PST in California, it's 11:00am MST in Tucson, Arizona, 12:00pm CST in Louisiana, and 1:00pm EST in Apopka, Florida. Hawaii and part of Alaska fall under Alaska-Hawaii Standard Time, while the rest of Alaska falls under Nome Standard Time. China has only one time zone, and there are lots of other exceptions. Airports often have a series of clocks disclosing the different times in different cities throughout the world.
Time has to start somewhere, and since time is an arbitrary designation set by humans, the agreed upon 0 hour occurs at 0 degrees longitude, the Prime Meridian. Also known as Greenwich Mean Time (GMT), Universal Time (UT), Zulu Time, and Zebra Time, each time zone of approximately 15 degrees longitude, moving eastward, is one hour later, or +1h.
It might be hard to believe a shepherds watch can tell time as reliably as a wristwatch. While they can't tell time to the second or minute, solar clocks can be fairly accurate down to the hour.
Always try to do things in chronological order; it is less confusing that way. Without time, everything would happen at once.
"What's a sundial in the shade?"
-- Benjamin Franklin