How many days prior to employment can I apply for TN?

Hi, all

There is my first post here..I am so glad that I found this website!

I have a quick question. I am a Canadian citizen studying in the States, Seattle, WA, particularly. I just finished my program and got a job offer. The letter of employment states that I am supposed to start on September 15th, 2010. I plan to drive up to the BC/Washington border to apply for my TN, but I don't know if there is a specific requirement on how many days prior to your employment can you apply. I guess sep5th would be safe, but I wanna get this over with asap, so I was hoping if sep1st or 2nd would do. Please help!!

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