Monthly数据类型

Monthly由来

最近在做关于智能财税的项目，大量用到了账期相关的数据操作。项目已有两年历史了，对于账期数据，前辈们用的是DateTime数据类型，即每个月的最后一天就是账期。而用DateTime来表达账期数据，确实让我人很困惑：

1. 概念不统一：
   DateTime是时间类型，而账期只跟年月相关，DateTime用在这里确实有点杀鸡用了宰牛刀，而且给人的理解和沟通造成了额外的误解。
2. 格式不统一：
   为了在数据传输和存储中达到数据的统一性，需要大量的字符串与日期的转换、日期格式的转换。
3. 浪费性能：
   DateTime的精确度是可以到毫秒级的，而我们的账期数据只需要精确到月，如：2018年1月账期。 所以DateTime是很影响运算性能和存储空间的。
4. 操作异常：
   由于账期是取月末日期，所以对每次接收了账期参数都要取月末值，以确保数据的准确性。而在实际开发中，任何一个疏忽都会引发表达偏差。

Monthly简介

Monthly是一个跟Datetime类似的，与月份相关的数据类型，适用于表达年月数据，如账单、账期、月刊等信息。

Monthly源码

using System.Collections.Generic;
using System.Runtime.InteropServices;
using System.Text.RegularExpressions;

namespace System
{
    /// <summary>
    /// 与月份相关的对象，如账单、账期、月刊、月报等
    /// </summary>
    [Serializable]
    [StructLayout(LayoutKind.Auto)]
    public struct Monthly : IComparable<Monthly>, IEquatable<Monthly>
    {
        private int _year;
        private int _month;

        #region Property
        /// <summary>
        ///  获取当前实例的年
        /// </summary>
        public int Year => _year;

        /// <summary>
        ///  获取当前实例的月
        /// </summary>
        public int Month => _month;

        /// <summary>
        /// 获取当前实例的年月标记值，如2018年1月记为 : 201801
        /// </summary>
        public int Dot => this._year * 100 + this._month;

        /// <summary>
        /// 获取当前实例从公元零年一月开始的月份累计值
        /// </summary>
        public int Tickes => this._year * 12 + this._month;

        /// <summary>
        ///  获取当前实例所在的季度
        /// </summary>
        public int Quarter => (this._month - 1) / 3 + 1;
        #endregion

        #region Ctor
        /// <summary>
        ///  以指定的年和月初始化Monthly实例。
        /// </summary>
        /// <param name="year"> 年（0 到 9999）</param>
        /// <param name="month"> 月（1 到 12）</param>
        public Monthly(int year, int month)
        {
            CheckYear(year);
            CheckMonth(month);
            this._year = year;
            this._month = month;
        }

        /// <summary>
        /// 获取以当前时间点为依据的新实例
        /// </summary>
        public static Monthly Current => new Monthly() { _year = DateTime.Now.Year, _month = DateTime.Now.Month };

        /// <summary>
        /// 获取当前时间点的上月为依据的新实例
        /// </summary>
        public Monthly Previous => Monthly.fromTickes(this.Tickes - 1);

        /// <summary>
        /// 获取当前时间点的下月为依据的新实例
        /// </summary>
        public Monthly Next => Monthly.fromTickes(this.Tickes + 1);

        /// <summary>
        /// 获取当前年份的一月为依据的新实例  
        /// </summary>
        public Monthly First => new Monthly() { _year = this._year, _month = 1 };

        /// <summary>
        /// 获取当前年份的十二月为依据的新实例
        /// </summary>
        public Monthly Last => new Monthly() { _year = this._year, _month = 12 };

        /// <summary>
        /// 获取Monthly的最小值实例
        /// </summary>
        public static Monthly MinValue => new Monthly() { _year = 0, _month = 1 };

        /// <summary>
        /// 获取Monthly的最大值实例
        /// </summary>
        public static Monthly MaxValue => new Monthly() { _year = 9999, _month = 12 };
        #endregion

        #region Method
        private static int yearOfDot(int dot) => dot / 100;

        private static int monthOfDot(int dot) => dot % 100;

        /// <summary>
        /// 获取当前实例的年月标记值，如2018年1月记为 : 201801
        /// </summary>
        /// <returns></returns>
        public int ToDot() => this.Dot;

        /// <summary>
        /// 以当前实例与years的和值为依据创建一个新实例
        /// </summary>
        public Monthly AddYears(int years) => Monthly.FromTickes(this.Tickes + years * 12);

        /// <summary>
        /// 以当前实例与months的和值为依据创建一个新实例
        /// </summary>
        public Monthly AddMonths(int months) => Monthly.FromTickes(this.Tickes + months);

        /// <summary>
        /// 判断当前实例的值与给定实例的值是否相等
        /// </summary>
        public bool Equals(Monthly other) => this.Tickes == other.Tickes;

        /// <summary>
        /// 获取当前实例与给定实例的月份差值
        /// </summary>
        public int SpanMonths(Monthly other) => this - other;

        /// <summary>
        /// 获取当前实例与DateTime实例的月份差值
        /// </summary>
        public int SpanMonths(DateTime date) => this.Tickes - date.Year * 12 - date.Month;

        /// <summary>
        /// 获取当前实例与给定实例的大小比较的结果标识
        /// </summary>
        /// <param name="other"></param>
        /// <returns>-1:小于other实例值 ； 0 等于other实例值 ； 1：大于other实例值</returns>
        public int CompareTo(Monthly other)
        {
            if (this.Tickes < other.Tickes) return -1;
            if (this.Tickes > other.Tickes) return 1;
            else return 0;
        }

        /// <summary>
        /// 以年月标记值创建一个Monthly新实例
        /// </summary>
        /// <param name="dot">格式：201801</param>
        /// <returns></returns>
        public static Monthly FromDot(int dot)
        {
            var year = yearOfDot(dot);
            var month = monthOfDot(dot);
            if (year < 0 || year > 9999 || month < 1 || month > 12)
                throw new ArgumentOutOfRangeException("dot", dot, "Please enter correct dot format such as \'201801\'.");

            return new Monthly
            {
                _year = yearOfDot(dot),
                _month = monthOfDot(dot)
            };
        }

        private static Monthly fromTickes(int tickes)
        {
            return new Monthly
            {
                _year = (tickes - 1) / 12,
                _month = tickes % 12 == 0 ? 12 : tickes % 12
            };
        }

        /// <summary>
        /// 以年月累计值创建一个Monthly新实例
        /// </summary>
        /// <param name="tickes">以公元零年一月为起点的月份计数值（1-120000）</param>
        public static Monthly FromTickes(int tickes)
        {
            if (tickes < 1 || tickes > 120000)
                throw new ArgumentOutOfRangeException("tickes", tickes, "The tickes must beteen 1 and 120000 .");
            return fromTickes(tickes);
        }

        /// <summary>
        /// 以DateTime实例创建一个Monthly新实例
        /// </summary>
        public static Monthly FromDate(DateTime time) => new Monthly() { _year = time.Year, _month = time.Month };

        /// <summary>
        /// 以诸如"2018/01"格式的字符串创建一个Monthly新实例
        /// </summary>
        /// <param name="s">"2018/01"格式的字符串</param>
        /// <param name="spliter">分隔符</param>
        public static Monthly FromString(string s)
        {
            if (string.IsNullOrEmpty(s))
                throw new Exception("The parameter cannot be null or empty.");

            var nums = Regex.Matches(s, "[0-9]+");
            if (nums.Count == 0)
                throw new Exception("Please give the correct parameters, such as '2018/01' .");

            if (nums.Count == 1)
                return new Monthly(0, Convert.ToInt32(nums[0].ToString().TrimStart('0')));
            else
                return new Monthly(Convert.ToInt32(nums[0].ToString().TrimStart('0')), Convert.ToInt32(nums[1].ToString().TrimStart('0')));
        }

        /// <summary>
        /// 获取一段时间内的Monthly数轴(包含开始与结束月份)
        /// </summary>
        /// <param name="from">开始月份</param>
        /// <param name="to">结束月份</param>
        /// <returns></returns>
        public static List<Monthly> Axis(Monthly from, Monthly to)
        {
            var result = new List<Monthly>();
            var span = from - to;
            var len = (span ^ (span >> 31)) - (span >> 31) + 1;
            for (int i = 0; i < len; i++)
            {
                if (span > 0) result.Add(from - i);
                else result.Add(from + i);
            }
            return result;
        }

        /// <summary>
        /// 获取给定时间段内的Monthly集合(包含开始与结束月份)
        /// </summary>
        /// <param name="from">开始月份</param>
        /// <param name="to">结束月份</param>
        /// <returns></returns>
        public static List<Monthly> Axis(int from, int to)
        {
            return Axis(Monthly.FromDot(from), Monthly.FromDot(to));
        }

        /// <summary>
        /// 检查year的合法性
        /// </summary>
        private static void CheckYear(int year)
        {
            if (year < 0 || year > 9999)
                throw new ArgumentOutOfRangeException("year", year, "The year must beteen 0 and 9999 .");
        }

        /// <summary>
        /// 检查month的合法性
        /// </summary>
        private static void CheckMonth(int month)
        {
            if (month < 1 || month > 12)
                throw new ArgumentOutOfRangeException("month", month, "The month must beteen 1 and 12 .");
        }
        #endregion

        #region Operator
        /// <summary>
        /// 以给定实例与months的和值创建一个新实例
        /// </summary>
        /// <param name="months">月分数</param>
        public static Monthly operator +(Monthly m, int months) => FromTickes(m.Tickes + months);

        /// <summary>
        /// 以给定实例与months的差值创建一个新实例
        /// </summary>
        /// <param name="months">月分数</param>
        public static Monthly operator -(Monthly m, int months) => FromTickes(m.Tickes - months);

        /// <summary>
        /// 获取当前实例与给定实例的月份差值
        /// </summary>
        public static int operator -(Monthly m1, Monthly m2) => m1.Tickes - m2.Tickes;

        /// <summary>
        ///获取当前实例与DateTime实例的月份差值
        /// </summary>
        public static int operator -(Monthly m, DateTime d) => m.SpanMonths(d);

        public static Monthly operator ++(Monthly m) => m + 1;

        public static Monthly operator --(Monthly m) => m - 1;

        /// <summary>
        ///判断m1是否等于m2
        /// </summary>
        public static bool operator ==(Monthly m1, Monthly m2) => m1.Tickes == m2.Tickes;

        /// <summary>
        /// 判断m1是否不等于m2
        /// </summary>
        public static bool operator !=(Monthly m1, Monthly m2) => m1.Tickes != m2.Tickes;

        /// <summary>
        /// 判断m1是否小于m2
        /// </summary>
        public static bool operator <(Monthly m1, Monthly m2) => m1.Tickes < m2.Tickes;

        /// <summary>
        /// 判断m1是否大于m2
        /// </summary>
        public static bool operator >(Monthly m1, Monthly m2)
        {
            return m1.Tickes > m2.Tickes; ;
        }

        /// <summary>
        /// 判断m1是否小于等于m2
        /// </summary>
        public static bool operator <=(Monthly m1, Monthly m2)
        {
            return m1.Tickes <= m2.Tickes; ;
        }

        /// <summary>
        /// 判断m1是否大于等于m2
        /// </summary>
        public static bool operator >=(Monthly m1, Monthly m2)
        {
            return m1.Tickes >= m2.Tickes; ;
        }

        /// <summary>
        /// 以年月标识的Monthly实例
        /// </summary>
        /// <param name="dot">格式：201801</param>
        public static implicit operator Monthly(int dot)
        {
            return Monthly.FromDot(dot);
        }
        #endregion

        #region Override
        /// <summary>
        /// 获取包含"Y、y、M、m"字符格式的自定义Monthly字符串
        /// </summary>
        /// <param name="format">
        /// 如：yyyy/mm ; yy/mm ; yyyy年mm月 ;YYYY-Mm...
        /// 不区分大小写
        /// </param>
        /// <returns></returns>
        public string ToString(string format = "yyyy/mm")
        {
            return Format(this, format);
        }

        /// <summary>
        /// 判断当前实例的值与给定实例的转换值是否相等
        /// </summary>
        public override bool Equals(object obj)
        {
            if (obj is null) throw new ArgumentNullException("obj", "The parameter cannot be null.");
            if (obj is Monthly) return this == (Monthly)obj;
            if (obj is DateTime) return this == Monthly.FromDate((DateTime)obj);
            throw new ArgumentException("The parameter must be System.DateTime type or System.Monthly type .", "obj");
        }

        public override int GetHashCode()
        {
            Int64 ticks = Tickes;
            return unchecked((int)ticks) ^ (int)(ticks >> 32);
        }

        private static string Format(Monthly m, string format)
        {
            string _y = m.Year.ToString();
            string _m = m.Month.ToString();
            format = format.ToLower();
            if (!(format.Contains("yyyy") || format.Contains("yyyy")) && !(format.Contains("mm") || format.Contains("m")))
                throw new ArgumentException("The format expression error. ", nameof(format));
            if (format.Contains("yyyy"))
                format = format.Replace("yyyy", m.Year < 10 ? $"0{_y}" : _y);
            else if (format.Contains("yy"))
                format = format.Replace("yy", m.Year < 10 ? $"0{_y}" : _y.PadLeft(4, '0').Substring(2));
            if (format.Contains("mm"))
                format = format.Replace("mm", m.Month < 10 ? $"0{_m}" : _m);
            else if (format.Contains("m"))
                format = format.Replace("m", _m.TrimStart('0'));
            return format;
        }
        #endregion
    }
}

测试

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace Pilipa.Utility.Test
{
    [TestClass]
    public class MonthlyTest
    {
        [TestMethod]
        public void TestProps()
        {
            var tar = DateTime.Now;
            Monthly plan = Monthly.FromDate(tar);

            Assert.AreEqual(Monthly.Current, new Monthly(tar.Year, tar.Month));

            Assert.AreEqual(plan.Year, tar.Year);
            Assert.AreEqual(plan.Month, tar.Month);
            Assert.AreEqual(plan.Dot, tar.Year * 100 + tar.Month);
            Assert.AreEqual(plan.Tickes, tar.Year * 12 + tar.Month);

            Assert.AreEqual(plan.First.ToDot(), tar.Year * 100 + 1);
            Assert.AreEqual(plan.Last.ToDot(), tar.Year * 100 + 12);


            Assert.AreEqual(plan.First.Previous.ToDot(), tar.AddYears(-1).Year * 100 + 12);
            Assert.AreEqual(plan.Last.Next.ToDot(), tar.AddYears(1).Year * 100 + 1);

            Assert.AreEqual(plan.Quarter, GetQuarter(tar.Month));

            Assert.AreEqual(Monthly.MinValue, new Monthly(0, 1));
            Assert.AreEqual(Monthly.MaxValue, new Monthly(9999, 12));
        }

        private int GetQuarter(int q)
        {
            if (new System.Collections.Generic.List<int>() { 1, 2, 3 }.Contains(q)) return 1;
            if (new System.Collections.Generic.List<int>() { 4, 5, 6 }.Contains(q)) return 2;
            if (new System.Collections.Generic.List<int>() { 7, 8, 9 }.Contains(q)) return 3;
            if (new System.Collections.Generic.List<int>() { 10, 11, 12 }.Contains(q)) return 4;
            return 0;
        }


        [TestMethod]
        public void TestMethods()
        {
            Monthly plan = 201801;
            var tar = new DateTime(2018, 1, 1);
            var tip = false;

            //Dot
            Assert.AreEqual(new Monthly(0, 11), 11);
            Assert.AreEqual(new Monthly(1, 1), 101);
            Assert.AreEqual(new Monthly(100, 12), 10012);
            Assert.AreEqual(new Monthly(2018, 12), 201812);

            //Tickes
            Assert.AreEqual(((Monthly)101).Tickes, 13);
            Assert.AreEqual(((Monthly)201811).Tickes, 2018 * 12 + 11);

            //加月
            Assert.AreEqual(plan.AddMonths(-1), 201712);
            Assert.AreEqual(plan.AddMonths(-23), 201602);
            Assert.AreEqual(plan.AddMonths(22), 201911);

            //加月(随机)
            for (int i = 0; i < 100; i++)
            {
                var rd = new Random(Guid.NewGuid().GetHashCode()).Next(100);
                Assert.AreEqual(plan.AddMonths(rd), Monthly.FromDate(tar.AddMonths(rd)));
                Assert.AreEqual(plan.AddMonths(rd).Dot, tar.AddMonths(rd).Year * 100 + tar.AddMonths(rd).Month);
            }

            //加年
            Assert.IsTrue(plan.AddYears(6) == 202401);
            Assert.IsTrue(plan.AddYears(-18) == 200001);

            //加年（异常）
            try { var m = Monthly.Current.AddYears(-3000); }
            catch (Exception e) { if (e.Message.Contains("beteen 1 and 120000")) { tip = true; } }
            Assert.IsTrue(tip);

            //月份差
            Assert.AreEqual(plan.SpanMonths(201711), 2);
            Assert.AreEqual(plan.SpanMonths(201902), -13);

            //比较大小
            Assert.AreEqual(plan.CompareTo(201801), 0);
            Assert.AreEqual(plan.CompareTo(201701), 1);
            Assert.AreEqual(plan.CompareTo(202001), -1);

            //构造
            Assert.AreEqual(Monthly.FromDot(3), 3);
            Assert.AreEqual(Monthly.FromTickes(13), 101);
            Assert.AreEqual(Monthly.FromDate(new DateTime(2018, 12, 12)), 201812);
            Assert.AreEqual(Monthly.FromString("2018/01"), 201801);
            Assert.AreEqual(Monthly.FromString("2018年01月"), 201801);
            Assert.AreEqual(Monthly.FromString("2018@01/01"), 201801);
            Assert.AreEqual(Monthly.FromString((new DateTime(2018, 1, 1)).ToString()), 201801);
            Assert.AreEqual(Monthly.FromString("3"), 3);

            //月份轴
            var axis = Monthly.Axis(201711, 201901);
            Assert.IsTrue(axis.Count == 15);
            Assert.AreEqual(axis[0], 201711);
            Assert.AreEqual(axis[3], 201802);
            Assert.AreEqual(axis[14], 201901);

            axis = Monthly.Axis(201812, 201712);
            Assert.IsTrue(axis.Count == 13);
            Assert.AreEqual(axis[0], 201812);
            Assert.AreEqual(axis[12], 201712);
            
            //异常
            tip = false;
            try { Monthly m = 201800; }
            catch (Exception e) { if (e.Message.Contains("correct dot format")) { tip = true; } }  //dot format
            Assert.IsTrue(tip);

            tip = false;
            try { Monthly m = Monthly.FromDot(13); }
            catch (Exception e) { if (e.Message.Contains("correct dot format")) { tip = true; } }  //13月
            Assert.IsTrue(tip);

            tip = false;
            try { Monthly m = Monthly.FromTickes(999999); }
            catch (Exception e) { if (e.Message.Contains("must beteen 1 and 120000")) { tip = true; } }  //越界
            Assert.IsTrue(tip);

            tip = false;
            try { Monthly m = Monthly.FromString(null); }
            catch (Exception e) { if (e.Message.Contains("null or empty")) { tip = true; } }  //IsNullOrEmpty
            Assert.IsTrue(tip);

            tip = false;
            try { Monthly m = Monthly.FromString("abc"); }
            catch (Exception e) { if (e.Message.Contains("parameters")) { tip = true; } }  //格式错误
            Assert.IsTrue(tip);

            tip = false;
            try { Monthly m = Monthly.FromString("88"); }
            catch (Exception e) { if (e.Message.Contains("must beteen")) { tip = true; } }  //越界
            Assert.IsTrue(tip);
        }

        [TestMethod]
        public void TestOps()
        {
            Monthly plan = 201801;
            var tar = Monthly.FromString("2018.01");

            Assert.AreEqual(plan + 12, 201901);
            Assert.AreEqual(plan - 13, 201612);

            Assert.AreEqual(plan - (Monthly)201701, 12);
            Assert.AreEqual(plan - (new DateTime(2017, 12, 12)), 1);

            Assert.AreEqual(--plan, 201712);
            Assert.AreEqual(++plan, 201801);

            Assert.IsTrue(plan == Monthly.FromDot(201801));
            Assert.IsTrue(plan != Monthly.FromDot(201802));

            Assert.IsTrue(plan >= Monthly.FromDot(201801));
            Assert.IsTrue(plan < Monthly.FromDot(201803));
        }

        [TestMethod]
        public void TestOvr()
        {
            Monthly plan = 201801;
            var tar = Monthly.FromString("2018.01");

            //哈希码(相同dot具有相同的哈希码)
            Assert.AreEqual(plan.GetHashCode(), tar.GetHashCode());
            tar++;
            Assert.AreNotEqual(plan.GetHashCode(), tar.GetHashCode());

            //格式化
            Assert.AreEqual(plan.ToString(), "2018/01");
            Assert.AreEqual(plan.ToString("yy/mm"), "18/01");
            Assert.AreEqual(Monthly.FromDot(501).ToString("yy/mm"), "05/01");
            Assert.AreEqual(plan.ToString("YYYY年m月"), "2018年1月");
            Assert.AreEqual(plan.ToString("公元YyYy年mM月,哈哈..."), "公元2018年01月,哈哈...");

            //比较相等
            Assert.IsTrue(plan.Equals(Monthly.FromDot(201801)));
            Assert.IsTrue(plan.Equals(new DateTime(2018, 1, 1)));
            Assert.IsTrue(plan.Equals((object)Monthly.FromDot(201801)));
            Assert.IsFalse(plan.Equals(Monthly.FromDot(201901)));
        }
    }
}

Monthly使用介绍

1.Monthly构造

  //创建一个“2018年1月”的账期
   Monthly m1 = 201801;
   Monthly m2 = new Monthly(2018, 1);
   Monthly m3 = Monthly.FromDate(new DateTime(2018, 1, 1));
   Monthly m4 = Monthly.FromDot(201801);
   Monthly m5 = Monthly.FromTickes(2018 * 12 + 1);
   Monthly m6 = Monthly.FromString("2018年01月");
   
   Monthly cur = Monthly.Current;	//当前时间实例
   Monthly min = Monthly.MinValue;	//Monthly最小实例
   Monthly max = Monthly.MaxValue;	//Monthly最大实例

2. Monthly属性

属性	说明
Year	获取当前实例的年
Month	获取当前实例的月
Dot	获取当前实例的年月标记值，如2018年1月记为 : 201801
Tickes	获取当前实例从公元零年一月开始的月份累计值
First	获取当前年份的一月为依据的新实例
Last	获取当前年份的十二月为依据的新实例
Previous	获取当前时间点的上月为依据的新实例
Next	获取当前时间点的下月为依据的新实例
Quarter	获取当前实例所在的季度

3.Monthly方法

• ToDot();
  说明：获取当前实例的年月标记值，如2018年1月记为 : 201801
  ​

• AddYears(int years)
  说明：以当前实例与years的和值为依据创建一个新实例

• AddMonths(int months)
  说明：以当前实例与months的和值为依据创建一个新实例
  ​

• Equals(Monthly other)
  说明：判断当前实例的值与给定实例的值是否相等
  ​

• Equals(object obj)
  说明：判断当前实例的值与给定实例的转换值是否相等，obj可以是DateTime类型
  ​

• SpanMonths(Monthly other)
  说明：获取当前实例与给定实例的月份差值
  ​

• SpanMonths(DateTime date)
  说明：获取当前实例与DateTime实例的月份差值
  ​

• CompareTo(Monthly other)
  说明：获取当前实例与给定实例的大小比较的结果标识， -1:小于other实例值 ； 0 等于other实例值 ； 1：大于other实例值
  ​

• List<Monthly> Axis(int from, int to)
  说明：获取一段时间内的Monthly数轴(包含开始与结束月份)
  ​

• List<Monthly> Axis(Monthly from, Monthly to)
  说明：同 List Axis(int from, int to)
  ​

• ToString(string format = "yyyy/mm")
  说明：获取包含"Y、y、M、m"字符格式的自定义Monthly字符串，format 格式如：yyyy/mm ; yy/mm ; yyyy年mm月 ;YYYY-Mm...，不区分大小写
  ​

示例：

   Monthly m = 201801;
   m.CompareTo(201701);            
   m.Equals(DateTime.Now);
   m.Equals(201701);
   m.SpanMonths(new DateTime(2017, 1, 1));
   m.SpanMonths(201701);

   m.ToString();
   m.ToString("yy/mm");
   Monthly.FromDot(501).ToString("yy/mm");
   m.ToString("YYYY年m月");
   m.ToString("公元YyYy年mM月,哈哈...");

4.Monthly操作符

Monthly支持+、- 、* 、/ 、> 、>= 、< 、<= 、++ 、-- 、== 、!= 运算符操作。

特别注意:-操作，他有operator -(Monthly m, int months)和operator -(Monthly m1, Monthly m2)两个重载版本，且方法功能不同，如果是第二个版本，则必须显式标注被减对象的数据类型，如m-(Monthly)201701

Github源码

参考:
https://referencesource.microsoft.com/#mscorlib/system/datetime.cs,df6b1eba7461813b 微软Datetime数据类型