BC/BCE, or Before Christ/Before Common Era, is years prior to the start of the Gregorian calendar, 2023 years ago. For XX BC, the bigger the number, the older it is. Ancient Egypt began around 3100 BCE, or 5,123 years ago.

AD/CE, or Anno Domini/Year of Our Lord/Common Era, is years after the Gregorian Calendar. Bigger number, more recent. The Aztec Empire began in the early 1400s, around 600 years ago.