计算两个时间之间的月份差

在做一个项目时，碰到的需要按月份进行统计，如果合同到期时间不在月初或者月末，需要计算月份开始时间（xxxx年xx月01日 00时00分00秒）与合同结束时间之间相差的月份。自己写了个方法，在此记录一下。
下面上代码：
  1  #region 对日期的操作
  2         /// <summary>
  3         /// 获取月份的天数
  4         /// </summary>
  5         /// <param name="month"></param>
  6         /// <returns></returns>
  7         private int MonthDayNumber(int year, int month)
  8         {
  9             int num = 0;
 10             switch (month)
 11             {
 12                 case 1:
 13                     num = 31;
 14                     break;
 15                 case 2:
 16                     if (CheckCommonOrLeapYear(year))
 17                         num = 29;
 18                     else num = 28;
 19                     break;
 20                 case 3:
 21                     num = 31;
 22                     break;
 23                 case 4:
 24                     num = 30;
 25                     break;
 26                 case 5:
 27                     num = 31;
 28                     break;
 29                 case 6:
 30                     num = 30;
 31                     break;
 32                 case 7:
 33                     num = 31;
 34                     break;
 35                 case 8:
 36                     num = 31;
 37                     break;
 38                 case 9:
 39                     num = 30;
 40                     break;
 41                 case 10:
 42                     num = 31;
 43                     break;
 44                 case 11:
 45                     num = 30;
 46                     break;
 47                 case 12:
 48                     num = 31;
 49                     break;
 50             }
 51             return num;
 52         }
 53         /// <summary>
 54         /// 判断年份是平年还是闰年
 55         /// </summary>
 56         /// <param name="year"></param>
 57         /// <returns>true:闰年，false：平年</returns>
 58         private bool CheckCommonOrLeapYear(int year)
 59         {
 60             bool flag = true;
 61             if (year % 4 == 0 && year % 100 != 0)
 62                 flag = true;
 63             else
 64             {
 65                 if (year % 400 == 0)
 66                     flag = true;
 67                 else
 68                     flag = false;
 69             }
 70             return flag;
 71         }
 72         /// <summary>
 73         /// 获取查询统计的月份区间的月份数
 74         /// </summary>
 75         /// <param name="startDate"></param>
 76         /// <param name="endDate"></param>
 77         /// <returns></returns>
 78         private int getSelectMonthNumber(DateTime startDate, DateTime endDate)
 79         {
 80             int monthNumber = 0;
 81             if (startDate.Year == endDate.Year)
 82             {
 83                 monthNumber = endDate.Month - startDate.Month + 1;
 84             }
 85             if (endDate.Year > startDate.Year)
 86             {
 87                 monthNumber = (endDate.Year - startDate.Year) * 12 + endDate.Month + 1 - startDate.Month;
 88             }
 89             return monthNumber;
 90         }
 91         /// <summary>
 92         /// 计算两个日期之间相差的月份
 93         /// </summary>
 94         /// <param name="startDate"></param>
 95         /// <param name="endDate"></param>
 96         /// <returns></returns>
 97         private double getSelectMonthNumber(string startDate, string endDate)
 98         {
 99             DateTime timeStart = Convert.ToDateTime(startDate);
100             DateTime timeEnd = Convert.ToDateTime(endDate);
101             double monthNumber = 0;
102             if (timeStart.Year == timeEnd.Year)
103             {
104                 if (timeEnd.Day >= timeStart.Day && timeEnd.Month >= timeStart.Month)
105                 {
106                     monthNumber = timeEnd.Month - timeStart.Month + (Convert.ToDouble(timeEnd.Day) - Convert.ToDouble(timeStart.Day)) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
107                 }
108                 else if (timeEnd.Day < timeStart.Day && timeEnd.Month > timeStart.Month)
109                 {
110                     if ((timeEnd.Month - timeStart.Month) > 1)
111                     {
112                         TimeSpan ts = timeEnd - timeStart.AddMonths(1);
113                         monthNumber = timeEnd.Month - timeStart.Month - 1 + Convert.ToDouble(ts.Days) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
114                     }
115                     else
116                     {
117                         TimeSpan ts = timeEnd - timeStart;
118                         monthNumber = Convert.ToDouble(ts.Days) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
119                     }
120                 }
121             }
122             if (timeEnd.Year > timeStart.Year)
123             {
124                 if (timeEnd.Month >= timeStart.Month)
125                 {
126                     if (timeEnd.Day >= timeStart.Day)
127                     {
128                         monthNumber = timeEnd.Month - timeStart.Month + (Convert.ToDouble(timeEnd.Day) - Convert.ToDouble(timeStart.Day)) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
129                     }
130                     else
131                     {
132                         TimeSpan ts = timeEnd - timeStart.AddMonths(11);
133                         monthNumber = timeEnd.Month - timeStart.Month - 1 + Convert.ToDouble(ts.Days) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
134                     }
135                 }
136                 else
137                 {
138                     if (timeEnd.Day >= timeStart.Day)
139                     {
140                         monthNumber = timeEnd.Month + 12 - timeStart.Month + Convert.ToDouble(timeEnd.Day - timeStart.Day) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
141                     }
142                     else {
143                         TimeSpan ts = timeEnd - timeStart.AddMonths(timeEnd.Month + 12 - timeStart.Month );
144                         monthNumber = timeEnd.Month + 12 - timeStart.Month + Convert.ToDouble(ts.Days) / Convert.ToDouble(MonthDayNumber(timeEnd.Year, timeEnd.Month));
145                     }
146                 }
147             }
148 
149             return monthNumber;
150         }
151         #endregion
下面贴上几张测试的图：
由于是实际业务中所使用的，调用方法前，就已经判定必须满足结束时间大于等于开始时间，所以在demo中没有加上结束时间大于开始时间的判断。