Are you a leader or a follower? If you are a follower, follow your leader's timing even if it doesn't match the music/count. It won't hurt your dance on time when you can, it's good practice for your follow skills, and, trust me, you will encounter many a leader who doesn't keep time throughout your dancing life!
If you are a leader and the follow isn't following your timing, try making your lead more definite (clearer, not more forceful).
If you feel you must say something, wait until the timing problem causes an obvious mess-up and say "I think we might be off time here." Notice the "we"--you don't want to seem to be blaming your partner.
Be patient. I doubt anyone is "refusing" to dance on time. Some people with little music experience really can't hear the beat, but they will learn over time. For others, it may be taking all their mental energy just to get their feet in the right places, with no attention left for doing it on time, yet. That too will change as they get more experience.
Be nice to these people. Making them feel bad about their dancing will just make them dance worse. And struggling at the beginning is not necessarily a predictor of how good a dancer someone will eventually be. I know a man who, when he first started dancing, couldn't keep time and it was widely predicted he would never amount to anything as a dancer. He kept at it though, and is now a very elegant dancer who routinely wins competitions. A little niceness is a good investment--one of those people may turn out to be your favorite partner in the long run.