我父母2018年2月得到绿卡，2018年4月离境回的中国，我父亲身体不好所以他们一直呆在中国。请问他们必须什么时候回美国才能保持绿卡？要在美国呆多久？
-They must return to US before April. Otherwise, if they stay in China for > one year, their green card will be void automatically unless they can prove something out of control to prevent them to return on time (this is very complicate).
-They must return to US before April. Otherwise, if they stay in China for > one year, their green card will be void automatically unless they can prove something out of control to prevent them to return on time (this is very complicate).
