父母B2 入境時給的離境日期是12月24日，如果逾期1-2天離境（25號或26號）對以後入境和綠卡申請有不好影響嗎？節前機票比節後貴好幾百刀。
-If they overstay (even if only one day), their B2 vis will be automatically invalid, and they have to apply for a new B2 visa to come to US again even if their current B2 visa does not expire yet. It does not affect the green card application. If they don't want to leave before the I-94 expire, they may just file I-539 to apply for extension before their I-94 expires. Then they can leave any time during the I-539 pending (keep the I-539 filing receipt and bring it with them when they come to US again using the current visa, just in case).
