code: public static DoubleFunction<AtomicLong> age(final Clock clock) {
      return new DoubleFunction<AtomicLong>() {
        @Override public double apply(double t) {
          return (clock.wallTime() - t) / 1000.0;
        }
      };
    }
docs: Returns a function that computes the age in seconds. The value passed into the function
  should be a timestamp in milliseconds since the epoch. Typically this will be the return
  value from calling {@link java.lang.System#currentTimeMillis()}.
  
  @param clock
  Clock used to get the current time for comparing with the passed in value.
  @return
  Function that computes the age.