code: public Object scheduleAt(final Date date,
                               final Runnable runnable)
                               throws IllegalArgumentException {
  
          if (date == null) {
              throw new IllegalArgumentException("date == null");
          } else if (runnable == null) {
              throw new IllegalArgumentException("runnable == null");
          }
  
          return this.addTask(date.getTime(), runnable, 0, false);
      }
docs: Causes the specified Runnable to be executed once in the background
  at the specified time.
  
  @param date time at which to execute the specified Runnable
  @param runnable the Runnable to execute.
  @return opaque reference to the internal task
  @throws IllegalArgumentException if date or runnable is null