From Sega Retro
After Waizz (or alternately, After War), commonly abbreviated AW, is a designation used to number years in the Phantasy Star series. It defines years after an epoch based on the year that the planet Palma was first united under supreme King Waizz Landale. Predictably, the years before the epoch are designated BW, or "Before Waizz". Both designations are placed before the year, such as in "AW 100" or "BW 100." In the English translation of PS1, this became "Space Century", although later English versions of the games used the AW denotation.
The origin of the calendar and the meaning of "AW" are revealed only in the Phantasy Star Compendium.