code: public long diffDayPeriods(Calendar end) {
          long endL   =  end.getTimeInMillis() +  end.getTimeZone().getOffset(  end.getTimeInMillis() );
          long startL = this.getTimeInMillis() + this.getTimeZone().getOffset( this.getTimeInMillis() );
          return (endL - startL) / MILLISECS_PER_DAY;
      }
docs: find the number of days from this date to the given end date.
  later end dates result in positive values.
  Note this is not the same as subtracting day numbers.  Just after midnight subtracted from just before
  midnight is 0 days for this method while subtracting day numbers would yields 1 day.
  @param end - any Calendar representing the moment of time at the end of the interval for calculation.
  
  @return The difference in days