Can a company withhold an employment bonus after its due date?

In my accepted offer letter, I was to recieve an employment bonus 'after one year of employment.' I was due to recieve this bonus on June 09. When I inquired about why I had not recieved my bonus, I was told that the company does not issue bonuses but once a year. This is NOT what I agreed to, nor was I told this BEFORE I was employed by this company. The offer letter CLEARLY states "bonus after one year of employment," which would have been June 09. Is there any legal grounds for me to demand my bonus before Jan 2010?