单线解决方案

首先，检查两个日期是否都在当前年份，如果不是，则获取全年的月份，然后从年初和年末添加月份。

DateTime dateFrom = new DateTime(2019, 2, 1);
DateTime dateTo = new DateTime(2021, 5, 25);

第一个月

var monthCount = dateFrom.Year != dateTo.Year ? ((dateTo.Year - dateFrom.Year - 1) * 12) + (13 - dateFrom.Month + dateTo.Month) : dateTo.Month - dateFrom.Month + 1;

结果= 28

没有第一个月

monthCount = dateFrom.Year != dateTo.Year ? ((dateTo.Year - dateFrom.Year - 1) * 12) + (12 - dateFrom.Month + dateTo.Month) : dateTo.Month - dateFrom.Month;

结果= 27

有3种情况:同一年，前一年和其他年份。

如果日期不重要的话……

public int GetTotalNumberOfMonths(DateTime start, DateTime end)
{
    // work with dates in the right order
    if (start > end)
    {
        var swapper = start;
        start = end;
        end = swapper;
    }

    switch (end.Year - start.Year)
    {
        case 0: // Same year
            return end.Month - start.Month;

        case 1: // last year
            return (12 - start.Month) + end.Month;

        default:
            return 12 * (3 - (end.Year - start.Year)) + (12 - start.Month) + end.Month;
    }
}

最精确的方法是以月为单位的分数返回差值:

private double ReturnDiffereceBetweenTwoDatesInMonths(DateTime startDateTime, DateTime endDateTime)
{
    double result = 0;
    double days = 0;
    DateTime currentDateTime = startDateTime;
    while (endDateTime > currentDateTime.AddMonths(1))
    {
        result ++;

        currentDateTime = currentDateTime.AddMonths(1);
    }

    if (endDateTime > currentDateTime)
    {
        days = endDateTime.Subtract(currentDateTime).TotalDays;

    }
    return result + days/endDateTime.GetMonthDays;
}

简单快速的解决方案，计算2个日期之间的总月份。 如果你只想得到不同的月份，而不计算From date中的月份-只需从代码中删除+1。

public static int GetTotalMonths(DateTime From, DateTime Till)
        {
            int MonthDiff = 0;

            for (int i = 0; i < 12; i++)
            {
                if (From.AddMonths(i).Month == Till.Month)
                {
                    MonthDiff = i + 1;
                    break;
                }
            }

            return MonthDiff;
        }

假设这个月的日期不相关(即2011.1.1和2010.12.31之间的差为1)，date1 > date2为正值，date2 > date1为负值

((date1.Year - date2.Year) * 12) + date1.Month - date2.Month

或者，假设你想要两个日期之间的“平均月”的大致数字，下面的方法应该适用于所有日期，但日期差异非常大。

date1.Subtract(date2).Days / (365.25 / 12)

注意，如果您要使用后一种解决方案，那么您的单元测试应该声明应用程序设计使用的最宽日期范围，并相应地验证计算结果。

更新(感谢Gary)

如果使用“平均月份”方法，“每年平均天数”的更准确数字是365.2425。

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {

        label3.Text = new DateDifference(Convert.ToDateTime("2018-09-13"), Convert.ToDateTime("2018-11-15")).ToString();
        label2.Text = new DateDifference(Convert.ToDateTime("2018-10-12"), Convert.ToDateTime("2018-11-15")).ToString();

        DateDifference oDateDifference = new DateDifference(Convert.ToDateTime("2018-11-12"));
       label1.Text  =   oDateDifference.ToString();

    }
}




public class DateDifference
{
    public DateTime start { get; set; }
    public DateTime currentDAte { get; set; }
    public DateTime origstart { get; set; }
    public DateTime origCurrentDAte { get; set; }

    int days { get; set; }
    int months { get; set; }
    int years { get; set; }

    public DateDifference(DateTime postedDate, DateTime currentDAte)
    {
        this.start = this.removeTime(postedDate);
        this.currentDAte = this.removeTime(currentDAte);
        this.origstart = postedDate;
        this.origCurrentDAte = currentDAte;

    }

    public DateDifference(DateTime postedDate)
    {
        DateTime currentDate_ = DateTime.Now;
        this.start = this.removeTime(postedDate);
        this.currentDAte = this.removeTime(currentDate_);
        this.origstart = postedDate;
        this.origCurrentDAte = currentDate_;
        if (start > this.currentDAte)
        {
            throw new Exception("Current date is greater than date posted");
        }
        this.compute();
    }

    void compute()
    {
        while (this.start.Year <= this.currentDAte.Year)
        {
            if (this.start.Year <= this.currentDAte.Year && (this.start.AddMonths(1) <= this.currentDAte))
            {
                ++this.months;
                this.start = this.start.AddMonths(1);
            }

            if ((this.start.Year == this.currentDAte.Year) && (this.start >= this.currentDAte.AddMonths(-1) && this.start <= this.currentDAte))
            {
                break;
            }
        }

        while (this.start.DayOfYear < this.currentDAte.DayOfYear)
        {
            ++this.days;
            this.start = start.AddDays(1);
        }

        if (this.months > 11)
        {
            while (this.months > 11)
            {
                ++this.years;
                this.months = months - 12;
            }
        }

    }


    public override string ToString()
    {
        if (this.start > this.currentDAte)
        {
            throw new Exception("Current date is greater than date posted");
        }
        String ret = this.ComposeTostring();
        this.reset();
        return ret;
    }

    private String ComposeTostring()
    {
        this.compute();
        if (this.years > 0)
        {
            if (this.months > 0)
            {
                if (this.days > 0)
                {
                    return String.Format("{0} year{1}, {2} month{3} && {4} Day{5} ago", this.years, plural(this.years), this.months, plural(this.months), this.days, plural(this.days));
                }
                return String.Format("{0} year{1}, {2} month{3} ago", this.years, plural(this.years), this.months, plural(this.months));
            }
            else
            {
                if (this.days > 0)
                {
                    return String.Format("{0} year{1},{2} day{3} ago", this.years, plural(this.years), this.days, plural(this.days));
                }

                return String.Format("{0} year{1} ago", this.years, plural(this.years));

            }
        }

        if (this.months > 0)
        {
            if (this.days > 0)
            {
                return String.Format("{0} month{1}, {2} day{3} ago", this.months, plural(this.months), this.days, plural(this.days));
            }
            else
            {
                return String.Format("{0} month{1} ago", this.months, plural(this.months));
            }
        }

        if ((this.origCurrentDAte - this.origstart).Days > 0)
        {
            int daysDiff = (this.origCurrentDAte - this.origstart).Days;
            this.origstart = this.origstart.AddDays(daysDiff);
            int HoursDiff = (this.origCurrentDAte - this.origstart).Hours;
            return String.Format("{0} day{1}, {2} hour{3} ago", daysDiff, plural(daysDiff), HoursDiff, plural(HoursDiff));

        }
        else if ((this.origCurrentDAte - this.origstart).Hours > 0)
        {
            int HoursDiff = (this.origCurrentDAte - this.origstart).Hours;
            this.origstart = this.origstart.AddHours(HoursDiff);
            int MinDiff = (this.origCurrentDAte - this.origstart).Minutes;
            return String.Format("{0} hour{1}, {2} minute{3} ago", HoursDiff, plural(HoursDiff), MinDiff, plural(MinDiff));
        }
        else if ((this.origCurrentDAte - this.origstart).Minutes > 0)
        {

            int MinDiff = (this.origCurrentDAte - this.origstart).Minutes;
            this.origstart = this.origstart.AddMinutes(MinDiff);
            int SecDiff = (this.origCurrentDAte - this.origstart).Seconds;
            return String.Format("{0} minute{1}, {2} second{3} ago", MinDiff, plural(MinDiff), SecDiff, plural(SecDiff));
        }
        else if ((this.origCurrentDAte - this.origstart).Seconds > 0)
        {
            int sec = (this.origCurrentDAte - this.origstart).Seconds;
            return String.Format("{0} second{1}", sec, plural(sec));
        }

        return "";
    }

    String plural(int val)
    {
        return (val > 1 ? "s" : String.Empty);
    }

    DateTime removeTime(DateTime dtime)
    {
        dtime = dtime.AddHours(-dtime.Hour);
        dtime = dtime.AddMinutes(-dtime.Minute);
        dtime = dtime.AddSeconds(-dtime.Second);
        return dtime;
    }

    public void reset()
    {

        this.days = 0;
        this.months = 0;
        this.years = 0;
        this.start = DateTime.MinValue;
        this.currentDAte = DateTime.MinValue;
        this.origstart = DateTime.MinValue;
        this.origCurrentDAte = DateTime.MinValue;
    }
}