code: public static Duration standardDays(long days) {
          if (days == 0) {
              return ZERO;
          }
          return new Duration(FieldUtils.safeMultiply(days, DateTimeConstants.MILLIS_PER_DAY));
      }
docs: Create a duration with the specified number of days assuming that
  there are the standard number of milliseconds in a day.
  <p>
  This method assumes that there are 24 hours in a day,
  60 minutes in an hour, 60 seconds in a minute and 1000 milliseconds in
  a second. This will be true for most days, however days with Daylight
  Savings changes will not have 24 hours, so use this method with care.
  <p>
  A Duration is a representation of an amount of time. If you want to express
  the concept of 'days' you should consider using the {@link Days} class.
  
  @param days  the number of standard days in this duration
  @return the duration, never null
  @throws ArithmeticException if the days value is too large
  @since 1.6