Unit testing time

We’ve all run across the problem of trying to unit test some code that is dependent on the current date. Such as:

public String elapsedTime(Date d) {
  final Date now = new Date(); 
  final long ms = now.getTime() - d.getTime(); 
  return "" + ms / 1000 + " seconds ago"; 
} 

How do you test something like this? (or something more sophisticated and useful)

I found the solution in a posting about mock objects. The trouble is that this code is depending on the Date class, and its behavior of getting the current time. Imagine a Clock interface:

interface Clock {
      Date getNow(); 
}  

and then a rewritten elapsed time:

  public String elapsedTime(Date d, Clock clock) { final Date
      now = clock.getNow(); final long ms = now.getTime() - d.getTime(); return "" + ms / 1000 +
      " seconds ago"; }`  

Obviously the Clock can be injected in other ways, but now we have a testable and extensible method.