Well, the US has been kind of using he SI system for a long time. The Metric Act (year 1866) was an important kilometerstone, and the customary units have been defined through the metric system since the 19th century. The practical conversion to SI units has been slightly slow, though.
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There are, however, some unfortunate inconsistencies in the US customary system. For example, time is measured in seconds (SI unit). …