I think that in a lot of ways we can do a lot to influence 'our time' from how we take care of ourselves, the situations we put ourselves in, all the way to simply making the choice not to give up. Of course we can only control ourselves; crime, nature, and accidents can all still happen.
My best friend was killed last year in a car crash. Needless to say, it was tragic. He was probably the best person that I've known. He dealt with so many hardships growing up. His life was really starting to look up for him, he had just gotten engaged, and then just like that, everything came to an end. If that was pre-ordained or destiny, then it seems like a sick and cruel joke.
So, I don't know the answer wether we have a certain time to die, but for the sake of life, I do hope that the answer is no. Either way though, every day we have is a gift, wether it just be because of random chance, or because it was decided that it was ours to have