code: private static <U extends IsoUnit> Item<U> replaceFraction(
          long amount,
          U unit
      ) {
  
          if (unit.equals(MILLIS)) {
              amount = MathUtils.safeMultiply(amount, MIO);
              unit = cast(NANOS);
          } else if (unit.equals(MICROS)) {
              amount = MathUtils.safeMultiply(amount, 1000L);
              unit = cast(NANOS);
          } else {
              return null;
          }
  
          return Item.of(amount, unit);
  
      }
docs: optional