简化一下模型，一般来说服务器返回的都是按时间顺序排好的数组(unsorted)，排序后如result所示。

    var result: Array<Any> = []
    /// 里面代表月份
    let unsorted = [9, 9, 9, 8, 8, 7, 6, 6, 5, 4, 4, 1]
    billSort(unsorted)
    /// 输出结果
    result = [[9, 9, 9], [8, 8], [7], [6, 6], [5], [4, 4], [1]]

关键代码：

var result: Array<Any> = []
/// 用于保存上一次排序的月份，以便接下来比较
var olderValue: Int?
func billSort(_ a: [Int]) {
    guard a.count > 1 else {
        result.append(a)
        return
    }

    olderValue = a.first
    /// 取出同月份的数据
    let currentMonth = a.filter{ $0 == olderValue }
    result.append(currentMonth)
    /// 取出不同月份的数据进行下一次比较
    let rest = a.filter{ $0 != olderValue }
    
    /// 判断是否还有未排序
    if rest.isEmpty == false {
        olderValue = rest.first
        billSort(rest)
    }
}

let unsorted = [9, 9, 9, 8, 8, 7, 6, 6, 5, 4, 4, 1]
billSort(unsorted)
print(result)
// result = [[9, 9, 9], [8, 8], [7], [6, 6], [5], [4, 4], [1]]

实际情况只需要将判定的条件换为是否同月。完整代码如下：

class BillModel {
    var timeStamp: Double?
    var money: Double?
    
    init(_ time: Double, money: Double) {
        self.timeStamp = time
        self.money = money
    }
}

func creatBill() -> [BillModel] {
    var currentTime = NSDate().timeIntervalSince1970
    var bills: [BillModel] = []
    for x in 0 ..< 10 {
        let bill = BillModel.init(currentTime, money: Double(x))
        bills.append(bill)
        currentTime -= 1000000
    }
    return bills
}

func isTheSameMonth(_ a: Double, b: Double) -> Bool {
    let aDate = Date.init(timeIntervalSince1970: a)
    let bDate = Date.init(timeIntervalSince1970: b)
    let aM = Calendar.current.dateComponents([.month], from: aDate)
    let bM = Calendar.current.dateComponents([.month], from: bDate)
    
    return aM.month == bM.month && aM.year == bM.year
}

var result: Array<Any> = []
var olderValue: BillModel?
func billSort(_ a: [BillModel]) {
    guard a.count > 1 else {
        result.append(a)
        return
    }
    
    olderValue = a.first
    let currentMonth = a.filter{ isTheSameMonth($0.timeStamp!, b: (olderValue?.timeStamp)!) }
    print(currentMonth)
    result.append(currentMonth)
    let rest = a.filter{ !isTheSameMonth($0.timeStamp!, b: (olderValue?.timeStamp)!) }
    
    if rest.isEmpty == false {
        olderValue = rest.first
        billSort(rest)
    }
}

let bills = creatBill()
billSort(bills)
print(result)

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