code: public long getDifferenceAsLong(ReadableInstant instant) {
          if (instant == null) {
              return getField().getDifferenceAsLong(getMillis(), DateTimeUtils.currentTimeMillis());
          }
          return getField().getDifferenceAsLong(getMillis(), instant.getMillis());
      }
docs: Returns the difference between this field property instant and the one
  passed in, in the units of this field. The sign of the difference
  matches that of compareTo. In other words, this field property's instant
  is the minuend.
  
  @param instant  the subtrahend, null means now
  @return the difference in the units of this field
  @see DateTimeField#getDifference