docs: public static final String getDurationUnits(final String duration) {
      if (duration == null || duration.isEmpty()) {
        throw new IllegalArgumentException("Duration cannot be null or empty");
      }
      int unit = 0;
      while (unit < duration.length() && 
          Character.isDigit(duration.charAt(unit))) {
        unit++;
      }
      final String units = duration.substring(unit).toLowerCase();
      if (units.equals("ms") || units.equals("s") || units.equals("m") || 
          units.equals("h") || units.equals("d") || units.equals("w") || 
          units.equals("n") || units.equals("y")) {
        return units;
      }
      throw new IllegalArgumentException("Invalid units in the duration: " + units);
    }