code: function startOf (unit = 'DD') {
    switch (unit) {
      case 'mm':
        return Miment(this.format('YYYY-MM-DD hh:mm:00'))
      case 'hh':
        return Miment(this.format('YYYY-MM-DD hh:00:00'))
      case 'DD':
        return Miment(this.format('YYYY-MM-DD 00:00:00'))
      case 'ww':
      case 'WW':
        return Miment(this.firstDayOfWeek().format('YYYY-MM-DD 00:00:00'))
      case 'MM':
        return Miment(this.format('YYYY-MM-01 00:00:00'))
      case 'YY':
      case 'YYYY':
        return Miment(this.format('YYYY-01-01 00:00:00'))
    }
  }
docs: 获取起始时间