How to calculate actual months difference (calendar year not approximation) between two given dates in C#?

Example: given two dates below, finish is always greater than or equal to start

start = 2001 Jan 01

finish = 2002 Mar 15

So from 2001 Jan 01 to the end of 2002 Feb

months = 12 + 2 = 14

For 2002 March

15/30 = 0.5

so grand total is 14.5 months difference.

It's very easy to work out by hand but how do I code it elegantly? At the moment I have the combination of a lot of if else and while loops to achieve what I want but I believe there are simpler solutions out there.

Update: the output needs to be precise (not approximation) for example: if start 2001 Jan 01 and finish 2001 Apr 16, the output should be 1 + 1 + 1= 3 (for Jan, Feb and Mar) and 16 / 31 = 0.516 month, so the total is 3.516.

Another example would be if I start on 2001 Jul 5 and finish on 2002 Jul 10, the output should be 11 month up to the end of June 2002, and (31-5)/31 = 0.839 and 10/31 = 0.323 months, so the total is 11 + 0.839 + 0.323 = 12.162.

I extended Josh Stodola's code and Hightechrider's code:

public static decimal GetMonthsInRange(this IDateRange thisDateRange)
{
    var start = thisDateR开发者_运维技巧ange.Start;
    var finish = thisDateRange.Finish;

    var monthsApart = Math.Abs(12*(start.Year - finish.Year) + start.Month - finish.Month) - 1;

    decimal daysInStartMonth = DateTime.DaysInMonth(start.Year, start.Month);
    decimal daysInFinishMonth = DateTime.DaysInMonth(finish.Year, finish.Month);

    var daysApartInStartMonth = (daysInStartMonth - start.Day + 1)/daysInStartMonth;
    var daysApartInFinishMonth = finish.Day/daysInFinishMonth;

    return monthsApart + daysApartInStartMonth + daysApartInFinishMonth;
}

---

I gave an int answer before, and then realized what you asked for a more precise answer. I was tired, so I deleted and went to bed. So much for that, I was unable to fall asleep! For some reason, this question really bugged me, and I had to solve it. So here you go...

static void Main(string[] args)
{
    decimal diff;

    diff = monthDifference(new DateTime(2001, 1, 1), new DateTime(2002, 3, 15));
    Console.WriteLine(diff.ToString("n2")); //14.45

    diff = monthDifference(new DateTime(2001, 1, 1), new DateTime(2001, 4, 16));
    Console.WriteLine(diff.ToString("n2")); //3.50

    diff = monthDifference(new DateTime(2001, 7, 5), new DateTime(2002, 7, 10));
    Console.WriteLine(diff.ToString("n2")); //12.16

    Console.Read();
}

static decimal monthDifference(DateTime d1, DateTime d2)
{
    if (d1 > d2)
    {
        DateTime hold = d1;
        d1 = d2;
        d2 = hold;
    }

    int monthsApart = Math.Abs(12 * (d1.Year-d2.Year) + d1.Month - d2.Month) - 1;
    decimal daysInMonth1 = DateTime.DaysInMonth(d1.Year, d1.Month);
    decimal daysInMonth2 = DateTime.DaysInMonth(d2.Year, d2.Month);

    decimal dayPercentage = ((daysInMonth1 - d1.Day) / daysInMonth1)
                          + (d2.Day / daysInMonth2);
    return monthsApart + dayPercentage;
}

Now I shall have sweet dreams. Goodnight :)

---

What you want is probably something close to this ... which pretty much follows your explanation as to how to calculate it:

var startofd1 = d1.AddDays(-d1.Day + 1);
var startOfNextMonthAfterd1 = startofd1.AddMonths(1);      // back to start of month and then to next month
int daysInFirstMonth = (startOfNextMonthAfterd1 - startofd1).Days;
double fraction1 = (double)(daysInFirstMonth - (d1.Day - 1)) / daysInFirstMonth;     // fractional part of first month remaining

var startofd2 = d2.AddDays(-d2.Day + 1);
var startOfNextMonthAfterd2 = startofd2.AddMonths(1);      // back to start of month and then to next month
int daysInFinalMonth = (startOfNextMonthAfterd2 - startofd2).Days;
double fraction2 = (double)(d2.Day - 1) / daysInFinalMonth;     // fractional part of last month

// now find whole months in between
int monthsInBetween = (startofd2.Year - startOfNextMonthAfterd1.Year) * 12 + (startofd2.Month - startOfNextMonthAfterd1.Month);

return monthsInBetween + fraction1 + fraction2;

NB This has not been tested very well but it shows how to handle problems like this by finding well known dates at the start of months around the problem values and then working off them.

While loops for date time calculations are always a bad idea: see http://www.zuneboards.com/forums/zune-news/38143-cause-zune-30-leapyear-problem-isolated.html

---

Depending on how exactly you want your logic to work, this would at least give you a decent approximation:

// 365 days per year + 1 day per leap year = 1461 days every 4 years
// But years divisible by 100 are not leap years
// So 1461 days every 4 years - 1 day per 100th year = 36524 days every 100 years
// 12 months per year = 1200 months every 100 years
const double DaysPerMonth = 36524.0 / 1200.0;

double GetMonthsDifference(DateTime start, DateTime finish)
{
    double days = (finish - start).TotalDays;
    return days / DaysPerMonth;
}

---

One way to do this is that you'll see around quite a bit is:

private static int monthDifference(DateTime startDate, DateTime endDate)
{
    int monthsApart = 12 * (startDate.Year - endDate.Year) + startDate.Month - endDate.Month;
    return Math.Abs(monthsApart);
}

However, you want "partial months" which this doesn't give. But what is the point in comparing apples (January/March/May/July/August/October/December) with oranges (April/June/September/November) or even bananas that are sometimes coconuts (February)?

An alternative is to import Microsoft.VisualBasic and do this:

    DateTime FromDate;
    DateTime ToDate;
    FromDate = DateTime.Parse("2001 Jan 01");
    ToDate = DateTime.Parse("2002 Mar 15");

    string s = DateAndTime.DateDiff (DateInterval.Month, FromDate,ToDate, FirstDayOfWeek.System, FirstWeekOfYear.System ).ToString();

However again:

The return value for DateInterval.Month is calculated purely from the year and month parts of the arguments

[Source]

---

Just improved Josh's answer

    static decimal monthDifference(DateTime d1, DateTime d2)
    {
        if (d1 > d2)
        {
            DateTime hold = d1;
            d1 = d2;
            d2 = hold;
        }

        decimal monthsApart = Math.Abs((12 * (d1.Year - d2.Year)) + d2.Month - d1.Month - 1);


        decimal daysinStartingMonth = DateTime.DaysInMonth(d1.Year, d1.Month);
        monthsApart = monthsApart + (1-((d1.Day - 1) / daysinStartingMonth));

        //  Replace (d1.Day - 1) with d1.Day incase you DONT want to have both inclusive difference.



        decimal daysinEndingMonth = DateTime.DaysInMonth(d2.Year, d2.Month);
        monthsApart = monthsApart + (d2.Day / daysinEndingMonth);


        return monthsApart;
    } 

---

The answer works perfectly and while the terseness of the code makes it very small I had to break everything apart into smaller functions with named variables so that I could really understand what was going on... So, basically I just took Josh Stodola's code and Hightechrider's mentioned in Jeff's comment and made it smaller with comments explaining what was going on and why the calculations were being made, and hopefully this may help someone else:

    [Test]
    public void Calculate_Total_Months_Difference_Between_Two_Dates()
    {
        var startDate = DateTime.Parse( "10/8/1996" );

        var finishDate = DateTime.Parse( "9/8/2012" );  // this should be now:


        int numberOfMonthsBetweenStartAndFinishYears = getNumberOfMonthsBetweenStartAndFinishYears( startDate, finishDate );


        int absMonthsApartMinusOne = getAbsMonthsApartMinusOne( startDate, finishDate, numberOfMonthsBetweenStartAndFinishYears );


        decimal daysLeftToCompleteStartMonthPercentage = getDaysLeftToCompleteInStartMonthPercentage( startDate );


        decimal daysCompletedSoFarInFinishMonthPercentage = getDaysCompletedSoFarInFinishMonthPercentage( finishDate );

        // .77 + .26 = 1.04
        decimal totalDaysDifferenceInStartAndFinishMonthsPercentage = daysLeftToCompleteStartMonthPercentage + daysCompletedSoFarInFinishMonthPercentage;


        // 13 + 1.04 = 14.04 months difference.
        decimal totalMonthsDifference = absMonthsApartMinusOne + totalDaysDifferenceInStartAndFinishMonthsPercentage;

        //return totalMonths;

    }

    private static int getNumberOfMonthsBetweenStartAndFinishYears( DateTime startDate, DateTime finishDate )
    {
        int yearsApart = startDate.Year - finishDate.Year;

        const int INT_TotalMonthsInAYear = 12;

        // 12 * -1 = -12
        int numberOfMonthsBetweenYears = INT_TotalMonthsInAYear * yearsApart;

        return numberOfMonthsBetweenYears;
    }

    private static int getAbsMonthsApartMinusOne( DateTime startDate, DateTime finishDate, int numberOfMonthsBetweenStartAndFinishYears )
    {
        // This may be negative i.e. 7 - 9 = -2
        int numberOfMonthsBetweenStartAndFinishMonths = startDate.Month - finishDate.Month;

        // Absolute Value Of Total Months In Years Plus The Simple Months Difference Which May Be Negative So We Use Abs Function
        int absDiffInMonths = Math.Abs( numberOfMonthsBetweenStartAndFinishYears + numberOfMonthsBetweenStartAndFinishMonths );

        // Subtract one here because we are going to use a perecentage difference based on the number of days left in the start month
        // and adding together the number of days that we've made it so far in the finish month.
        int absMonthsApartMinusOne = absDiffInMonths - 1;

        return absMonthsApartMinusOne;
    }

    /// <summary>
    /// For example for 7/8/2012 there are 24 days left in the month so about .77 percentage of month is left.
    /// </summary>
    private static decimal getDaysLeftToCompleteInStartMonthPercentage( DateTime startDate )
    {
        // startDate = "7/8/2012"

        // 31
        decimal daysInStartMonth = DateTime.DaysInMonth( startDate.Year, startDate.Month );

        // 31 - 8 = 23 
        decimal totalDaysInStartMonthMinusStartDay = daysInStartMonth - startDate.Day;

        // add one to mark the day as being completed. 23 + 1 = 24
        decimal daysLeftInStartMonth = totalDaysInStartMonthMinusStartDay + 1;

        // 24 / 31 = .77 days left to go in the month
        decimal daysLeftToCompleteInStartMonthPercentage = daysLeftInStartMonth / daysInStartMonth;

        return daysLeftToCompleteInStartMonthPercentage;
    }

    /// <summary>
    /// For example if the finish date were 9/8/2012 we've completed 8 days so far or .24 percent of the month
    /// </summary>
    private static decimal getDaysCompletedSoFarInFinishMonthPercentage( DateTime finishDate )
    {
        // for septebmer = 30 days in month.
        decimal daysInFinishMonth = DateTime.DaysInMonth( finishDate.Year, finishDate.Month );

        // 8 days divided by 30 = .26 days completed so far in finish month.
        decimal daysCompletedSoFarInFinishMonthPercentage = finishDate.Day / daysInFinishMonth;

        return daysCompletedSoFarInFinishMonthPercentage;
    }

---

This solution calculates whole months and then adds the partial month based on the end of the time period. This way it always calculates full months between the dates' day-of-month and then calculates the partial month based on the number of remaining days.

public decimal getMonthDiff(DateTime date1, DateTime date2) {
    // Make parameters agnostic
    var earlyDate = (date1 < date2 ? date1 : date2);
    var laterDate = (date1 > date2 ? date1 : date2);

    // Calculate the change in full months
    decimal months = ((laterDate.Year - earlyDate.Year) * 12) + (laterDate.Month - earlyDate.Month) - 1;

    // Add partial months based on the later date
    if (earlyDate.Day <= laterDate.Day) {
        decimal laterMonthDays = DateTime.DaysInMonth(laterDate.Year, laterDate.Month);
        decimal laterPartialMonth = ((laterDate.Day - earlyDate.Day) / laterMonthDays);
        months += laterPartialMonth + 1;
    } else {
        var laterLastMonth = laterDate.AddMonths(-1);
        decimal laterLastMonthDays = DateTime.DaysInMonth(laterLastMonth.Year, laterLastMonth.Month);
        decimal laterPartialMonth = ((laterLastMonthDays - earlyDate.Day + laterDate.Day) / laterLastMonthDays);
        months += laterPartialMonth;
    }
    return months;
}

---

The calculation below is one that is according the way the Dutch Tax Authority wants months calculated. This means that when the starts day is for example feb 22, march 23 should be result in something above 1 and not just something like 0.98.

    private decimal GetMonthDiffBetter(DateTime date1, DateTime date2)
    {
        DateTime start = date1 < date2 ? date1 : date2;
        DateTime end = date1 < date2 ? date2 : date1;

        int totalYearMonths = (end.Year - start.Year) * 12;
        int restMonths = end.Month - start.Month;
        int totalMonths = totalYearMonths + restMonths;

        decimal monthPart = (decimal)end.Day / (decimal)start.Day;
        return totalMonths - 1 + monthPart;
    }`

---

This should get you where you need to go:

DateTime start = new DateTime(2001, 1, 1);
DateTime finish = new DateTime(2002, 3, 15);
double diff = (finish - start).TotalDays / 30;

---

the framework as a TimeSpan object that is a result of subtracting two dates.

the subtraction is already considering the various option of February(28/29 days a month) so in my opinion this is the best practice after you got it you can format it the way you like best

        DateTime dates1 = new DateTime(2010, 1, 1);
        DateTime dates2 = new DateTime(2010, 3, 15);
        var span = dates1.Subtract(dates2);
        span.ToString("your format here");

---

    private Double GetTotalMonths(DateTime future, DateTime past)
    {
        Double totalMonths = 0.0;

        while ((future - past).TotalDays > 28 )
        {
            past = past.AddMonths(1);
            totalMonths += 1;
        }

        var daysInCurrent = DateTime.DaysInMonth(future.Year, future.Month);
        var remaining = future.Day - past.Day;

        totalMonths += ((Double)remaining / (Double)daysInCurrent);
        return totalMonths;
    }