朋友虽然在过去的5年中没有在中国居住连续超过6个月，但是过去5年里在美国总共居住的时间有限，可能不足30个月。
-Then, he probably is not qualified for N-400 yet until he meets the requirement of continuous residency (30 momths in the last 5 years).
另外，他在美国居住的日子，移民局那里有精确的记录可查吗？
-I think USCIS system should have the record, but he may not be allowed to check it. He should have recorded his travel by himself.
因为离境除了航班记录，护照上似乎没有离境记录，他中美往返多了以后自己都搞不清楚究竟每年在美国境内居住了多长时间？
