I am due for bonus from my current employer "X" in the month of January 2013. This amount comprises of about 15% of my Annual Gross Pay (AGP) and is variable. I have a new offer in hand and the new company "Y" wants me to join in November 2012. I've already negotiated with Company "Y", which is a large and well-known company, and received an offer, so it is unlikely I can negotiate further. In comparison, company "X" is a much lesser known company (they work in a niche domain), but I have been working here for 7 years. I am planning to put down my papers, but would it be reasonable to ask my current employer to pay the bonus on a pro rata basis? If so, how can I put it across them in a polite way?
P.S: People who join "X" in the middle of the year get the bonus based on proportional period of time they have worked in that year, but I do not know the case for people who leave the company.