text: import pad from './pad'
  
  /**
   * @param {String} value YYYY-MM format
   * @param {Number} sign -1 or +1
   */
  export default (value, sign) => {
    const [year, month] = value.split('-').map(v => 1 * v)
  
    if (month + sign === 0) {
      return `${year - 1}-12`
    } else if (month + sign === 13) {
      return `${year + 1}-01`
    } else {
      return `${year}-${pad(month + sign)}`
    }
  }
  