I am with the current company (in Canada, with LMIA) as a skilled worker since Sep 2015. At that time, I signed the offer with the duration of 2 years. Since I will need a new one, in order to get 600 points, with the wording permanent, full-time and non-seasonal, I am wondering which date should be there? Like I signed it before I started working (before Sep 2015), or current date will be fine?
Just to be clear, the job offer letter is not something YOU sign, it is something your employer signs.
The letter should be "recent" - Current date is fine, assuming you are applying soon. Doesn't matter if you have already started this position - as long as you are on a temporary work permit, it is essentially temporary employment. And if your employer already considers you permanent, they shouldn't have any issues with preparing a new letter).
Just to be clear, the job offer letter is not something YOU sign, it is something your employer signs.
The letter should be "recent" - Current date is fine, assuming you are applying soon. Doesn't matter if you have already started this position - as long as you are on a temporary work permit, it is essentially temporary employment. And if your employer already considers you permanent, they shouldn't have any issues with preparing a new letter).