in a week 表示在将来的一周之内,而in a week's time 整表示未来整整一周之后.

in a week=in a week's time
不用纠结啦
He will be back in a week.
He will be back in a week's time.
他将在一周之后回来.
当然按一楼说的 “in a week是一周之后,in a week's time是一周之内.”也OK
比如 I'll finish the homework in a week's time.我将在一周以内完成作业.
看具体语境