I think you mean that it is a letter to your employer.
So, do you already have the job? If so, why don't you just tell the person that? If you don't have the job then you are talking about, not an employer but a potential employer.
My first appointment date was 12-03-2004 at Apollo Hospital, and I was under probation for a period of three year from the first date of appointment to 12-03-2007. I successfully completed my probation period, and my position was made permanent. After that I was registered with the nursing council.