请教8老师， 我父母的绿卡申请递交上去半年了，在 等待中。 想请教下， 一旦拿到绿卡， 一年中需在美国至少呆半年， 时间应该怎么算？是从拿到绿卡才开始算， 还是在今年一年内在美国呆够半年就可以 （包括还没拿到绿卡的时间）？谢谢！
-After ther get the green card, they need to live in US permanently. Of curse, they can visit China any time, and can stay in China for about 180 days occasionally. But if they go back and forth between China and US too often, they will have trouble at the port of entry.
