code: public static DayDuration valueOf(String duration) {
  		int spacePos = duration.indexOf(' ');
  		if(spacePos == -1) throw new IllegalArgumentException("Space not found in duration: " + duration);
  		return getInstance(
  			Integer.parseInt(duration.substring(0, spacePos)),
  			Unit.valueOfUnit(duration.substring(spacePos + 1))
  		);
  	}
docs: Parses a duration compatible with results of toString.
  Supports:
  <ol>
  <li># day</li>
  <li># days</li>
  <li># week</li>
  <li># weeks</li>
  <li># month</li>
  <li># months</li>
  <li># year</li>
  <li># years</li>
  </ol>
  
  @see  #toString()