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How can I get the difference between two times which are being given as decimal format [duplicate]

This question already has answers here:
Converting a custom date format (string) to a datetime
(3 answers)
Closed 1 year ago.
I am trying to find the difference between two times . I have two variable in decimal datatype to store the both time value. for example
decimal intime = 57.30
decimal breaktime= 8.36
decimal nettime = intime - breaktime
The nettime value should be 48.54. ie,
57:30 - 8:36 = 48:54 will get in excel. How can I get the same result in C# using the variable intime and breaktime.

Quick and dirty:
TimeSpan.FromMinutes((int)intime).Add(TimeSpan.FromSeconds(intime * 100 - (int)intime * 100)) - TimeSpan.FromMinutes((int)breaktime).Add(TimeSpan.FromSeconds(breaktime * 100 - (int)breaktime * 100))
You could extract the decimal to TimeSpan conversion in a method.

public static decimal SecondsToDecimal(decimal input)
{
var whole = Math.Floor(input);
return Math.Round(whole + ((input - whole) * (100.0M / 60)),2);
}
public static decimal DecimalToSeconds(decimal input)
{
var whole = Math.Floor(input);
return Math.Round(whole + ((input - whole) * (60.0M / 100)),2);
}
public static void Main()
{
decimal intime = 57.30M;
decimal breaktime= 8.36M;
var nettime = SecondsToDecimal(intime) - SecondsToDecimal(breaktime);
Console.WriteLine(DecimalToSeconds(nettime)); //produces 48.54
}
See it work here:
https://dotnetfiddle.net/xYLBIs

You could try with this:
public static int[] getMinAndSec(double i)
{
int t = (int) ( 100 * i);
int min = t/100;
int sec = (t%100);
return new int[] {min, sec};
}
public static void getDifference() {
int[] A = getMinAndSec(intime);
int[] B = getMinAndSec(breaktime);
if(A[1] >= B[1])
nettime = A[0] - B[0] + (A[1] - B[1]) * 0.01;
else
{
nettime = A[0] - B[0] - 1 + (60 - (A[1] + B[1] - 60)) * 0.01;
}
}
It converts your decimal to seconds and milliseconds, and then substracts them according to their value to give you back a time where nothing goes over 60.

when calculating time, it doesn't account for a different day

I have this code that calculates time. but when it goes in to the next day it sort of resets and doesn't keep going forward. what im trying to do is when it goes past 12AM, to keep counting the hours and print the result as 10, and not go back to 3
string entry_T = dt.Rows[0][3].ToString();
int delta_day = Int32.Parse(entry_T.Substring(8, 2)) - Int32.Parse(DT.Substring(8, 2));
int st_h = Int32.Parse(entry_T.Substring(11, 2));
int ed_h = Int32.Parse(DT.Substring(11, 2));
int st_m = Int32.Parse(entry_T.Substring(14, 2));
int ed_m = Int32.Parse(DT.Substring(14, 2));
double delta_T = ((ed_h + 24 * delta_day) * 60 + ed_m - st_h * 60 - st_m) / 60.0;
if (delta_T <= 1) print = 3;
else if (delta_T <= 2) print = 5;
else if (delta_T <= 3) print = 7;
else if (delta_T <= 4) print = 8;
else print = 10;
return cost;

You can subtract 2 DateTimes to get a TimeSpan.
var dt1 = DateTime.Parse(your_string);
var timeSpan = DateTime.Now - dt1;
//timeSpan will tell you the difference, corrected for anomalies like leap years.
see: https://learn.microsoft.com/en-us/dotnet/api/system.timespan?view=netcore-3.1

Milliseconds to the highest possible time value before reaching 0,xx

I would like to write a converter from milliseconds to the highest possible time value before reaching a 0,x value.
Let me clarify this with examples.
Let's assume you have 1500ms this should result in 1,5secs, because its the highest possible digit value not resulting in 0,x.
So different examples would be
10ms = 10,0ms
100ms = 100,0ms
1000ms = 1,0sec
10000ms = 10,0sec
100000ms = 1,6min
1000000ms = 16,0min
10000000ms = 2,7hours
(The method should more or less be endless, so from hours to days, to weeks, to months, to years, to decades and so on...)
Is there a .net method for this?

Something like the following
public static string ConversionMethod(UInt64 ms)
{
// change output format as needed
string format = "######.###";
var cutoffs = new List<UInt64>() {
1000, // second
60000, // minute
3600000, // hour
86400000, // day
604800000, // week = day * 7
2592000000, // month = day * 30
31536000000, // year = day * 365
315360000000, // decade = year * 10
3153600000000, // century = decade * 10 (100 years)
31536000000000, // millenia = century * 10 (1000 years)
31536000000000000 // megayear = year * 100000
// 18446744073709551615 // UInt64 MaxValue
// 31536000000000000000 // gigayear = year * 100000000
};
var postfix = new List<String>() {
"second",
"minute",
"hour",
"day",
"week",
"month",
"year",
"decade",
"century",
"millenia",
"megayear"
};
// The above are listed from smallest to largest for easy reading,
// but the comparisons need to be made from largest to
// smallest (in the loop below)
cutoffs.Reverse();
postfix.Reverse();
int count = 0;
foreach (var cutoff in cutoffs)
{
if (ms > cutoff)
{
return ((decimal)((decimal)ms / (decimal)cutoff)).ToString(format) + " " + postfix[count];
}
count++;
}
return ms + " ms";
}
Conversion for the fraction is a bit dirty, might want to clean that up. Also, you'll have to decide how you want to handle leap years (and leap seconds), etc.

While not the final solution, maybe TimeSpan can help you achieve what you are looking for.
It is to be noted however, TimeSpan supports only up to TotalDays.
var timespan = TimeSpan.FromMilliseconds(1500);
var seconds = timespan.TotalSeconds; // equals: 1.5

It seems the TimeSpan class is the closest thing that meets your need, but clearly it's not exactly what you want. My take on it would look something like this:
public static string ScientificNotationTimespan(int milliseconds)
{
var timeSpan = new TimeSpan(0, 0, 0, 0, milliseconds);
var totalDays = timeSpan.TotalDays;
if (totalDays < 7)
{
if (timeSpan.TotalDays > 1) return timeSpan.TotalDays.ToString() + " days";
if (timeSpan.TotalHours > 1) return timeSpan.TotalHours.ToString() + " hours";
if (timeSpan.TotalMinutes > 1) return timeSpan.TotalMinutes.ToString() + " minutes";
if (timeSpan.TotalSeconds > 1) return timeSpan.TotalSeconds.ToString() + " seconds";
return milliseconds.ToString() + "milliseconds";
}
var weeks = totalDays / 7;
//How long is a month? 28, 29, 30 or 31 days?
var years = totalDays / 365;
if (years < 1) return weeks.ToString() + " weeks";
var decades = years / 10;
if (decades < 1) return years.ToString() + " years";
var centuries = decades / 10;
if (centuries < 1) return decades.ToString() + " decades";
var millenia = centuries / 10;
if (millenia < 1) return centuries.ToString() + " centuries";
return millenia.ToString() + " millenia";
}

Here is solution for years, months using DateTime and Gregorian calendar (meaning leap years, calendar months). Then it uses the TimeSpan solution as already submitted.
static string ToMostNonZeroTime(long ms) {
const int hundretsNanosecondsInMillisecond = 10000;
long ticks = (long)ms * hundretsNanosecondsInMillisecond;
var dt = new DateTime(ticks);
if((dt.Year - 1) > 0) { // starts with 1
double daysToYear = (dt.DayOfYear - 1) * 1.0 / (DateTime.IsLeapYear(dt.Year) ? 366 : 365);
daysToYear += dt.Year - 1;
return $"{daysToYear:0.0} years";
}
if((dt.Month - 1) > 0) {
double daysToMonth = (dt.Day - 1) * 1.0 / DateTime.DaysInMonth(dt.Year, dt.Month);
daysToMonth += dt.Day - 1;
return $"{daysToMonth:0.0} months";
}
// can use TimeSpan then:
var ts = TimeSpan.FromMilliseconds(ms);
if(ts.TotalDays >= 1)
return $"{ts.TotalDays:0.0} days";
if(ts.TotalHours >= 1)
return $"{ts.TotalHours:0.0} hours";
if(ts.TotalMinutes >= 1)
return $"{ts.TotalMinutes:0.0} minutes";
if(ts.TotalSeconds >= 1)
return $"{ts.TotalSeconds:0.0} seconds";
return $"{ms} milliseconds";
}
It prints
100ms: 100 milliseconds
1000ms: 1.0 seconds
10000ms: 10.0 seconds
100000ms: 1.7 minutes
1000000ms: 16.7 minutes
10000000ms: 2.8 hours
100000000ms: 1.2 days
1000000000ms: 11.6 days
20000000000ms: 19.6 months
200000000000ms: 6.3 years
Have a look at https://ideone.com/QZHOM4

Date difference in years using C# [duplicate]

This question already has answers here:
How do I calculate someone's age based on a DateTime type birthday?
(74 answers)
Closed 6 years ago.
How can I calculate date difference between two dates in years?
For example: (Datetime.Now.Today() - 11/03/2007) in years.

I have written an implementation that properly works with dates exactly one year apart.
However, it does not gracefully handle negative timespans, unlike the other algorithm. It also doesn't use its own date arithmetic, instead relying upon the standard library for that.
So without further ado, here is the code:
DateTime zeroTime = new DateTime(1, 1, 1);
DateTime a = new DateTime(2007, 1, 1);
DateTime b = new DateTime(2008, 1, 1);
TimeSpan span = b - a;
// Because we start at year 1 for the Gregorian
// calendar, we must subtract a year here.
int years = (zeroTime + span).Year - 1;
// 1, where my other algorithm resulted in 0.
Console.WriteLine("Yrs elapsed: " + years);

Use:
int Years(DateTime start, DateTime end)
{
return (end.Year - start.Year - 1) +
(((end.Month > start.Month) ||
((end.Month == start.Month) && (end.Day >= start.Day))) ? 1 : 0);
}

We had to code a check to establish if the difference between two dates, a start and end date was greater than 2 years.
Thanks to the tips above it was done as follows:
DateTime StartDate = Convert.ToDateTime("01/01/2012");
DateTime EndDate = Convert.ToDateTime("01/01/2014");
DateTime TwoYears = StartDate.AddYears(2);
if EndDate > TwoYears .....

If you need it for knowing someone's age for trivial reasons then Timespan is OK but if you need for calculating superannuation, long term deposits or anything else for financial, scientific or legal purposes then I'm afraid Timespan won't be accurate enough because Timespan assumes that every year has the same number of days, same # of hours and same # of seconds).
In reality the length of some years will vary (for different reasons that are outside the scope of this answer). To get around Timespan's limitation then you can mimic what Excel does which is:
public int GetDifferenceInYears(DateTime startDate, DateTime endDate)
{
//Excel documentation says "COMPLETE calendar years in between dates"
int years = endDate.Year - startDate.Year;
if (startDate.Month == endDate.Month &&// if the start month and the end month are the same
endDate.Day < startDate.Day// AND the end day is less than the start day
|| endDate.Month < startDate.Month)// OR if the end month is less than the start month
{
years--;
}
return years;
}

var totalYears =
(DateTime.Today - new DateTime(2007, 03, 11)).TotalDays
/ 365.2425;
Average days from Wikipedia/Leap_year.

int Age = new DateTime((DateTime.Now - BirthDateTime).Ticks).Year;
To calculate the elapsed years (age), the result will be minus one.
var timeSpan = DateTime.Now - birthDateTime;
int age = new DateTime(timeSpan.Ticks).Year - 1;

Here is a neat trick which lets the system deal with leap years automagically. It gives an accurate answer for all date combinations.
DateTime dt1 = new DateTime(1987, 9, 23, 13, 12, 12, 0);
DateTime dt2 = new DateTime(2007, 6, 15, 16, 25, 46, 0);
DateTime tmp = dt1;
int years = -1;
while (tmp < dt2)
{
years++;
tmp = tmp.AddYears(1);
}
Console.WriteLine("{0}", years);

It's unclear how you want to handle fractional years, but perhaps like this:
DateTime now = DateTime.Now;
DateTime origin = new DateTime(2007, 11, 3);
int calendar_years = now.Year - origin.Year;
int whole_years = calendar_years - ((now.AddYears(-calendar_years) >= origin)? 0: 1);
int another_method = calendar_years - ((now.Month - origin.Month) * 32 >= origin.Day - now.Day)? 0: 1);

I implemented an extension method to get the number of years between two dates, rounded by whole months.
/// <summary>
/// Gets the total number of years between two dates, rounded to whole months.
/// Examples:
/// 2011-12-14, 2012-12-15 returns 1.
/// 2011-12-14, 2012-12-14 returns 1.
/// 2011-12-14, 2012-12-13 returns 0,9167.
/// </summary>
/// <param name="start">
/// Stardate of time period
/// </param>
/// <param name="end">
/// Enddate of time period
/// </param>
/// <returns>
/// Total Years between the two days
/// </returns>
public static double DifferenceTotalYears(this DateTime start, DateTime end)
{
// Get difference in total months.
int months = ((end.Year - start.Year) * 12) + (end.Month - start.Month);
// substract 1 month if end month is not completed
if (end.Day < start.Day)
{
months--;
}
double totalyears = months / 12d;
return totalyears;
}

public string GetAgeText(DateTime birthDate)
{
const double ApproxDaysPerMonth = 30.4375;
const double ApproxDaysPerYear = 365.25;
int iDays = (DateTime.Now - birthDate).Days;
int iYear = (int)(iDays / ApproxDaysPerYear);
iDays -= (int)(iYear * ApproxDaysPerYear);
int iMonths = (int)(iDays / ApproxDaysPerMonth);
iDays -= (int)(iMonths * ApproxDaysPerMonth);
return string.Format("{0} år, {1} måneder, {2} dage", iYear, iMonths, iDays);
}

I found this at TimeSpan for years, months and days:
DateTime target_dob = THE_DOB;
DateTime true_age = DateTime.MinValue + ((TimeSpan)(DateTime.Now - target_dob )); // Minimum value as 1/1/1
int yr = true_age.Year - 1;

If you're dealing with months and years you need something that knows how many days each month has and which years are leap years.
Enter the Gregorian Calendar (and other culture-specific Calendar implementations).
While Calendar doesn't provide methods to directly calculate the difference between two points in time, it does have methods such as
DateTime AddWeeks(DateTime time, int weeks)
DateTime AddMonths(DateTime time, int months)
DateTime AddYears(DateTime time, int years)

DateTime musteriDogum = new DateTime(dogumYil, dogumAy, dogumGun);
int additionalDays = ((DateTime.Now.Year - dogumYil) / 4); //Count of the years with 366 days
int extraDays = additionalDays + ((DateTime.Now.Year % 4 == 0 || musteriDogum.Year % 4 == 0) ? 1 : 0); //We add 1 if this year or year inserted has 366 days
int yearsOld = ((DateTime.Now - musteriDogum).Days - extraDays ) / 365; // Now we extract these extra days from total days and we can divide to 365

Works perfect:
internal static int GetDifferenceInYears(DateTime startDate)
{
int finalResult = 0;
const int DaysInYear = 365;
DateTime endDate = DateTime.Now;
TimeSpan timeSpan = endDate - startDate;
if (timeSpan.TotalDays > 365)
{
finalResult = (int)Math.Round((timeSpan.TotalDays / DaysInYear), MidpointRounding.ToEven);
}
return finalResult;
}

Simple solution:
public int getYearDiff(DateTime startDate, DateTime endDate){
int y = Year(endDate) - Year(startDate);
int startMonth = Month(startDate);
int endMonth = Month(endDate);
if (endMonth < startMonth)
return y - 1;
if (endMonth > startMonth)
return y;
return (Day(endDate) < Day(startDate) ? y - 1 : y);
}

This is the best code to calculate year and month difference:
DateTime firstDate = DateTime.Parse("1/31/2019");
DateTime secondDate = DateTime.Parse("2/1/2016");
int totalYears = firstDate.Year - secondDate.Year;
int totalMonths = 0;
if (firstDate.Month > secondDate.Month)
totalMonths = firstDate.Month - secondDate.Month;
else if (firstDate.Month < secondDate.Month)
{
totalYears -= 1;
int monthDifference = secondDate.Month - firstDate.Month;
totalMonths = 12 - monthDifference;
}
if ((firstDate.Day - secondDate.Day) == 30)
{
totalMonths += 1;
if (totalMonths % 12 == 0)
{
totalYears += 1;
totalMonths = 0;
}
}

Maybe this will be helpful for answering the question: Count of days in given year,
new DateTime(anyDate.Year, 12, 31).DayOfYear //will include leap years too
Regarding DateTime.DayOfYear Property.

The following is based off Dana's simple code which produces the correct answer in most cases. But it did not take in to account less than a year between dates. So here is the code that I use to produce consistent results:
public static int DateDiffYears(DateTime startDate, DateTime endDate)
{
var yr = endDate.Year - startDate.Year - 1 +
(endDate.Month >= startDate.Month && endDate.Day >= startDate.Day ? 1 : 0);
return yr < 0 ? 0 : yr;
}

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 = thisDateRange.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;
}