Letter of Rec. for future University application I'm getting my Software Engineer degree in one year here in my country, and I want to continue my studies with a Master degree in the US. 
Since here in my country is not a common practice to ask or write such letters, I'm not sure about some details.
I have two main concerns about them: 

1) How important is the written date of the letter? (It's OK to get one now, dated accordingly, if I will be submitting it in say, 2 years?)

2) Must the letter be always addressed? (It's OK to be generic, not addressed at anyone in particular? -At this moment I don't know what University I would like to apply-)

I'm asking those two questions because currently I'm developing an important software for the local Police (the 911 emergency line) and I think this could be a nice vouch for my technical skills, however since I'm changing jobs in a few weeks it's possible I won't stay in touch anymore with the Chief, or maybe this Chief won't be around in the future to ask him. This also because now I can ask for it and get it in paper, with the official letterhead and seal/stamp, so that will be a proof of authenticity.

Let's say my idea is to get this letter now and save it for later. Of course this would be a field expertise type of letter.
Thanks!
Your letters should preferably be written by one of your professors, that will hopefully be aware of the protocols on how to write them. Also, usually you don't get access to your letters, but provide the university you are applying to with the contact of recommendation writers that have agreed to write your letter. The idea is that letters are supposed to be blind, so the recommender can freely express their opinions about you, and also make sure you haven't tampered with it.

I would advise against getting a letter from the police chief. He can certify that you did that job (and that should be attached to your application), but he doesn't have the technical skills to evaluate your job beyond "it works", and he cannot put you in perspective compared with other students. Plus, industry letters are usually a string of cold facts:


  This person worked here from November to April developing the system C.


Whereas an academic letter is on the line of:


  I strongly recommend the student for the program because... as he showed working on ...
  [And some paragraphs more saying how awesome you are].