it's siad that in the 3rd century a chinese man wrote about

in the 3rd century 在本句中作时间状语.
在英语中,时间状语的位置是最灵活的,可出现在句首、句中或句尾.区别就是,在句首时往往用逗号隔开,其余则不用.
以 at last 为例：
At last,he finished the work.(句首)
He at last finished the work.（句中）
He finished the work at last.（句尾）