In the Middle Ages, Europeans began to date events according to the estimated birthday of Christ: "BC" (before Christ) and "AD" (anno domini, "in the year of our lord").
In modern times, Western commerce spread the Christian calender all over the globe. In order to accommodate other religions, the newer terms "BCE" and "CE" are now used ("before the Common Era" and "Common Era").
The years 0-99 count as the 1st century; the 1600s (the years 1600-1699) count as the 17th century.