Calendars and clocks being somewhat more common than socket wrenches or twist drills, the installed base for the second and the twelvemonth likely implies more hysteresis there than for the 10 mm as opposed to the 3/8" socket. Conversions are relatively unimportant, serving more as a device for math teachers to turn students off than for aiding in acceptance of metric units.
The replacement of the Gregorian calendar and the implementation of a new system of weights and measures are two instances of rational constructivism that the French revolutionaries attempted. The idea was to use the decimal system so as to make calculation easier for time, distances, weight, etc. The meter (one ten-millionth of the distance from the Equator to the North Pole), the gram, and the liter were defined as the standard units of weights and measures (see here and here).
An article in The Economist this week relates the failed attempt during the French Revolution to implement the decimal system for time keeping. Days in the Republican Calendar were divided into ten hours, each hour into 100 minutes, each minute into 100 seconds. It didn’t catch on and Napoleon abolished the new system in 1805. He didn’t abolish the system of weights and measures however, which eventually became the International System of Units (SI) and was adopted by every nation but three (Burma, Liberia, and the United States).
The Economist article doesn’t tell us much about the interesting side of the story. Strangely it laments that SI was not adopted for the keeping of time and for the calendar, but it doesn’t explain why it failed. Why all this happened is an important question for those who regard rules as an evolutionary process in the Hayekian sense. Many libertarians in the US hate the metric system, as it represents the pinnacle of top-down state-imposed rules. Interestingly, the metric system never caught on in the US when it did in Canada and other ex-Anglo colonies such as Australia and New Zealand (as well as the rest of the world). This probably goes to show the deep cultural divide that exists between American society and the rest of the Common Wealth, especially with regard to the role of government. The costs associated with the imposition of a change in measurement units are huge. Not only people need to grasp what a meter represents compared to a foot, but also every machine that functions with the old units need to be changed or adapted.
But for all of the careful analysis of why science uses cgs or mks, which is two-thirds of the French system, in its work, the real reason for metric's failure to catch on in the United States, despite being the official system of measurement (a Reconstruction era act of Congress) is the decimals at the foul poles during baseball's experiment with traditional and international measurements. That metric's advocates often come off as elitists incapable of carrying a gallon of water for a patternmaker or plowing six furlongs doesn't help the case.