How did you receive your orders? For example, did someone hand the orders to you? If yes, can that person write a statement specifying when you received the orders?
Have you talked with someone in the military (such as your officer or the military's attorney office)?
Also, are you on a month-to-month rental or for a longer term rental? The 7 day notice is in RCW 59.18.220 (Termination of tenancy for a specified time – Armed forces exception). If you are month-to-month, RCW 59.18.200 applies. The statutes are here: http://apps.leg.wa.gov/rcw/default.aspx?Cite=59 .
All you can do is give the apartment the orders within seven days of when you received them. Write a cover letter and explain that. Your landlord will have to allow you to leave and is prohibited from taking a judgment against you under these circumstances. Wishing you all the best - Elizabeth Powell