You are going to have to take this one step at a time.  If you have copies of all the documents that you signed and you claim that none of them mention an agreed upon time period then your employer will have to show proof of the agreement.  You are claiming that all your negotiations have been in writing - application and written offer - so you will state that his assertion has to be in writing in order to be valid here.  If he states that it was an oral agreement then he will have to prove it.  You are going to have to refute his claim and wait until a  determination is made.  If it is in your favor then I am sure that your employer will appeal.  If it is in his favor then you will appeal. And you may need to consult an employment attorney.  Good luck.