Either 180天 or 183天 is OK. Nothing affected.

怎么机票代理说往返机票/不能超过180天呢?
-I think it is because B2 holder normally is only allowed to stay in US for 6 months unless he extends the B2 status.

签证有效期6个月,入境给半年,如何买机票往返日期?
-签证有效期签证有效期6个月 means your parents need to enter US within 6 months with the visa, and it has nothing to do with how long they will stay in US.

入境给半年, 能用父母探亲报税?
-I do not know.

所有跟帖： 

• Thank863211老师 ,183天 is OK,I'll定票, Because worry BOUT算滞留,就麻烦了. -pinklemontea- ♀ (1 bytes) () 05/06/2010 postreply 06:33:32

• 滞留for several days does not hurt anything. -863211- ♂ (0 bytes) () 05/06/2010 postreply 06:37:42

• Thank863211老师 So appreciate! Good day:) Then I can book ticket f -pinklemontea- ♀ (178 bytes) () 05/06/2010 postreply 07:06:47

• Fine. -863211- ♂ (0 bytes) () 05/06/2010 postreply 09:54:37

• 863211老师, Good morning! Have a nice day:)) -pinklemontea- ♀ (381 bytes) () 05/06/2010 postreply 07:10:16

• 回复：863211老师, Good morning! Have a nice day:)) -863211- ♂ (94 bytes) () 05/06/2010 postreply 09:55:56