code: function _get (unit) {
    switch (unit) {
      case 'YYYY': return this.getFullYear()
      case 'MM': return this.getMonth() + 1
      case 'DD': return this.getDate()
      case 'hh': return this.getHours()
      case 'mm': return this.getMinutes()
      case 'ss': return this.getSeconds()
      case 'ww':
      case 'WW': return this.getDay()
      case 'SSS': return this.getMilliseconds()
    }
    return '00'
  }
docs: 获取