请问签证到底允许提前多少天？

考虑签证但是碰到一个困惑的问题：

http://chinese.usembassy-china.org.cn/five_secrets.html
申请学生签证不能在报到时间90天之前申请，因为签证法规定，签证官不能在学校开课90天之前发放签证。

http://shanghai.usembassy-china.org.cn/student_visas.html

Applicants for U.S. student visas should be aware that the Consular Section may not issue visas to qualified applicants more than 120 days in advance of the start date listed on their I-20 forms. Students may apply more than 120 days in advance, but will not be able to receive visas, if qualified, until no more than 120 days before their start dates.

我是湖南人，在北京签证。开学时间是9月10号的话，最早什么时候可以签证？5月还是6月？

谢谢！

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签证专区