“Eighteen hundreds,” “sixteen hundreds” and so forth are notexactly errors; the problem is that they are used almostexclusively by people who are nervous about saying“nineteenth century” when, after all, the years in thatcentury begin with the number eighteen. This should besimple: few people are unclear about the fact that this isthe twenty-first century even though our dates begin withtwenty. For most dates you can just add one to the thirddigit from the right in a year and you’ve got the number of its century. Ittook a hundred years to get to the year 100, so the nexthundred years, which are named “101,” “102,” etc. were inthe second century. This also works BC. The four hundreds BCare the fifth century BC. Using phrases like “eighteenhundreds” is a signal to your readers that you are weak inmath and history alike.