Programming puzzle: dates

Does your API have date utility methods? If so, the answer to the following quiz may surprise you a little. Suppose we have a method that subtracts two dates and returns the result in milliseconds.

import java.util.Date;

public long subtract(Date d1, Date d2) {
  if(d1 == null || d2 == null) {
    throw new IllegalArgumentException();
  }
  return d1.getTime() - d2.getTime();
}

The question is where the problem lies and how to fix it.