Trying to figure out what to call a century can be pretty tricky, especially when we make a distinction between BCE and CE. The way we mark time is based upon a fixed date presumed to be the birth of Jesus Christ. Although this way of marking time is based in religious calendar systems, it is increasingly secularized. Some people use the terms BC ("Before Christ,") and AD ("After Death," or anno domini, "in the year of our Lord,"), but most scholarly publications will use BCE (Before Current Era) and CE (Current Era) to distinguish between these times.
In all that came after the beginning of the Common Era, we count time in a progressive manner moving forward. Every year, we simply add one year onto the date we were using previously, and every century we add one in the hundreds place. Simple enough, right? Well, it gets tricky when we are trying to figure out what to call a century. At first reckoning, we might think that the 1st Century refers to the years 100-199 CE. It's easy to make this mistake because the number one is in the hundreds place and there is no number in the thousands place. When we say the 1st Century, though, we're actually referring to the years 1-99 CE. This means the year 1 CE came during the 1st Century CE.
In determining how to call a century from before the shift to the current era, we must work "backwards!" Think of BCE time as a countdown to point zero. Though a year zero never existed, for the purposes of this question we could consider the moment of transition between BCE and CE to be "point zero." The farther back we go in BCE time, the greater temporal distance from point zero, and so the larger the number. Just like with dating centuries in CE, the number of the century is not really representative of the number in the hundreds place. The 1st Century BCE is still the years 1-99 BCE; not 100-199. This means that the year 2012 BCE took place in the 21st Century BCE.
A handy way to figure out "when" years and centuries occurred and how to call them is to add or subtract one from the hundreds place.
For example, if you have the year 1492 and want to know what century this occurred in, add one to the hundreds place. This tells us that 1492 occurred during the 15th Century.
Similarly, if you know that something occurred during the 15th Century, but you're not sure what years this might have been, just subtract one from the hundreds place. This tells us that the 15th Century occurred during the years 1400-1499.