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.