code: static PlainDate doAdd(
          CalendarUnit unit,
          PlainDate context,
          long amount,
          int policy
      ) {
  
          switch (unit) {
              case MILLENNIA:
                  return doAdd(
                      CalendarUnit.MONTHS,
                      context,
                      MathUtils.safeMultiply(amount, 12 * 1000),
                      policy);
              case CENTURIES:
                  return doAdd(
                      CalendarUnit.MONTHS,
                      context,
                      MathUtils.safeMultiply(amount, 12 * 100),
                      policy);
              case DECADES:
                  return doAdd(
                      CalendarUnit.MONTHS,
                      context,
                      MathUtils.safeMultiply(amount, 12 * 10),
                      policy);
              case YEARS:
                  return doAdd(
                      CalendarUnit.MONTHS,
                      context,
                      MathUtils.safeMultiply(amount, 12),
                      policy);
              case QUARTERS:
                  return doAdd(
                      CalendarUnit.MONTHS,
                      context,
                      MathUtils.safeMultiply(amount, 3),
                      policy);
              case MONTHS:
                  long months =
                      MathUtils.safeAdd(context.getEpochMonths(), amount);
                  return PlainDate.fromEpochMonths(
                      context,
                      months,
                      context.dayOfMonth,
                      policy);
              case WEEKS:
                  return doAdd(
                      CalendarUnit.DAYS,
                      context,
                      MathUtils.safeMultiply(amount, 7),
                      policy);
              case DAYS:
                  return addDays(context, amount);
              default:
                  throw new UnsupportedOperationException(unit.name());
          }
  
      }
docs: <p>Additionsmethode. </p>
  
  @param   unit        calendar unit
  @param   context     calendar date
  @param   amount      amount to be added
  @param   policy      overflow policy
  @return  result of addition