Patterns for deriving state gist10

code.js

class TimeRangeStore {
  @observable
  days;

  constructor(days){
    this.days = days;
  }     
  
  @computed
  get formatted(){
    const {days} = this;
    return days > 14? `${Math.floor(days / 7)} weeks`:days>1?`${days} days`:'1 day'
  }
  
  @action
  set formatted(value){
      const [count,unit] = value.split(" ");
      const days = unit === "weeks"? count * 7: count;
      this.days = days;
  } 
}