code: public static Date offset(Long offsetInMillis) {
      DateTimeUtils.setCurrentMillisOffset(offsetInMillis);
      return new Date(DateTimeUtils.currentTimeMillis());
    }
docs: Moves the clock by the given offset and keeps it running from that point
  on.
  
  @param offsetInMillis
  the offset to move the clock by
  @return the new 'now'