Before it became the symbol of a program stalling on your PC, the hourglass spent centuries as the representation of mortality and an emblem of the sciences. Much more than a symbol, of course, it also kept track of time in the pre-Swatch Era. While the hourglass seems like the kind of primitive tool used by the ancients, its origins are surprisingly somewhat recent — unless its true history has been obscured by the very sands of time.
It has long been thought that hourglasses were certainly an ancient device, thanks especially to an ancient Roman bas-relief that seemed to include one. Recently, however, it was determined that the part of the bas-relief with the hourglass on it was only added in the 16th century.
In fact, the first authentic evidence of hourglasses did not appear until 1338 AD, when Ambrosio Lorenzetti painted a fresco with the personification of Temperance holding an hourglass (image 4). Later that century, hourglasses were mentioned in written documents, including a 1345 sales receipt for items bought by the clerk of an English ship, and the inventory taken at the 1380 death of King Charles V of France. So from these three examples, it is clear that by the early 14th century, hourglasses were in common usage by everyone from sailors to kings, and had already taken on symbolic values as a device of measurement.
It makes sense that a ship clerk would have purchased an hourglass, since it is a strong possibility that hourglasses were first developed for maritime use (image 5). Before the 14th century, time was measured in unequal hours, based on the durations of day and night on each date. On a ship, however, the measurement of equal hours would have been necessary for calculating the distance traveled (since distance = rate x time). Sand-filled hourglasses would have been preferable to water clocks (the most ancient form of clock, which functions similarly to the hourglass) because, if suspended, they would be relatively unaffected by the motion of the ocean.
So basically all the evidence points to the hourglass being invented around 1000-1100 AD, during that era's great advances in maritime navigation. This dating gives the hourglass roughly enough time to become widely used and to enter the material record around 1300.
Not everyone, however, is convinced.
It does, after all, seem bizarre that the hourglass would have been invented around the same time as the mechanical clock and the magnetic compass, two examples of advanced technological prowess compared to the decidedly analog hourglass. The sandglass's similarity to different types of water clock also raises some questions. The water clock was in common usage throughout Egypt, Greece, and China as early as 1500 BC. It's easy to imagine that the brilliant engineers of those great ancient cultures would have thought to use sand (which is, in some places, a much more abundant resource) instead of water. (Though the best hourglasses were not filled with regular sand, which is coarse and susceptible to humidity; common materials, at least during the Renaissance, were marble dust, pulverized tin or lead, or crushed eggshells.)
The other compelling argument in favor of an earlier origin for hourglasses is the fact that they were so prevalent a symbol in the visual arts starting in the 1300s AD. A symbol of Vanitas, or the futility and transience of earthly pursuits, hourglasses are visible in many Renaissance paintings as a emblem of human mortality (image 6). Death and Time were often depicted as old men carrying a scythe and an hourglass, a stark reminder of man's inexorable march toward the grave (image 7). Because of these morbid associations, the hourglass was also a symbol commonly used on pirate flags, a warning message to enemy ships about what awaited them if they chose to attack (image 8). Of course, it was still primarily a measure of time, and so also symbolized empiricism, popping up in images of astronomers and engineers as emblems of mathematical truth (image 9). So the question remains, could this object have permeated trans-European culture so thoroughly in only a few short centuries?
Of course, we may never know, unless genuine evidence older than 1338 turns up somewhere. In the meantime, the hourglass is still favored as a desktop accessory (image 10), whether for its associations with Renaissance culture, pirate culture, or with the inexorable passage of time.
Images: 1 Marc Newsom's Ikepod hourglass, via Josh Spear; 2 Early American hourglass found in Maine, via Sotheby's; 3 English and Scottish 19th and 20th c. hourglasses, via Christie's; 4 Temperance, depicted in fresco by Ambrosio Lorenzetti, via Wikimedia Commons; 5 Frontispiece of a Dutch 1608 book about navigation called Licht der Zeevaert, or The Light of Navigation, engraving by Willem Janszoon Blaeu, via Wikipedia; 6 Vanitas, by Philippe de Champaigne (c.1671), via Wikimedia Commons; 7 Knight, Death and the Devil by Albrecht Dürer, 1513-14, at the Metropolitan Museum of Art; 8 Blackbeard's flag, via Elizabethan-Era.org.uk; 9 Astronomer by Candlelight by Gerrit Dou (late 1650s), via the Getty; 10 Hourglasses at CB2, $10-30.
Sources: There is a lot written on hourglasses, much of it based on outdated ideas. The two sources I found most helpful were hourglasses.com and onlineclock.net, who present the 14th century and earlier theories, respectively.
If you want an hourglass to call your own, there are several options beyond the occasional antique one that comes up at auction houses. CB2 sells their green sand versions for $10-30. There are often ones that come up on eBay, including this brass 15-minute timer for $36 or this elegant 60-minute timer for $46. For a crisper look, Rain Collection has an assortment of simple, modern hourglasses in either black or white sand for $25-29.