Get the number of months between two DateTimes - vb.net

Question

mnth = DateDiff(DateInterval.Month, 8/30/2012, 10/1/2012)
gives mnth = 2. But when we look, there is only 32 days between these dates. I am expecting a result mnth=1 as there is only 32 days between these days.
Pls help..
In my scenario i can consider a 15+ days to be a month but if it is less than 15, it should'nt be considered.

Answer 1

To get the number of complete months you can do different things depending on your interpretation,
Public Function CompleteMonthsBetweenA( _
ByVal start As DateTime, _
ByVal end As DateTime) As Integer
Dim invertor = 1
If (start > end) Then
Dim tmp = end
end = start
start = tmp
invertor = -1
End If
Dim diff = ((end.Year - start.Year) * 12) + end.Month - start.Month
If start.Day > end.Day Then
Return (diff - 1) * invertor
Else
Return diff * invertor
End If
End Function
With this function the number of complete months between 31/05/2011 (dd/mm/yy) and 30/06/2011 is 0 but between 30/06/2011 and 31/07/2011 is 1. Which may or may not be what you expect.
Public Function CompleteMonthsBetweenB( _
ByVal start As DateTime, _
ByVal end As DateTime) As Integer
Dim invertor = 1
If (start > end) Then
Dim tmp = end
end = start
start = tmp
invertor = -1
End If
Dim diff = ((end.Year - start.Year) * 12) + end.Month - start.Month
Dim startDaysInMonth = DateTime.DaysInMonth(start.Year, start.Month)
Dim endDaysInMonth = DateTime.DaysInMonth(end.Year, end.Month)
If (start.Day / startDaysInMonth) > (end.Day / endDaysInMonth) Then
Return (diff - 1) * invertor
Else
Return diff * invertor
End If
End Function
With this function the ratio Day / DaysInMonth is taken so the relative completion of the two months can be assessed.
Public Function CompleteMonthsBetweenC( _
ByVal start As DateTime, _
ByVal enddate As DateTime) As Integer
Dim invertor = 1
If (start > enddate) Then
Dim tmp = enddate
enddate = start
start = tmp
invertor = -1
End If
Dim diff = ((enddate.Year - start.Year) * 12) + enddate.Month - start.Month
Dim remainingDays = _
(DateTime.DaysInMonth(start.Year, start.Month) - start.Day) + enddate.Day
If remainingDays < 15 Then
Return (diff - 1) * invertor
Else
Return diff * invertor
End If
End Function
This function only rounds down if the surplus days are less than the magic number 15, which I think is what you are asking for in your update.
Public Function CompleteMonthsBetweenD( _
ByVal start As DateTime, _
ByVal end As DateTime) As Integer
Return end.Subtract(start).TotalDays \ 30.436875
End Function
This function takes the simpler approach of dividing the total number of days by the average number of days per month in the Gregorian Calendar.

Answer 2

The difference in months is calculated without regard to the day component of the dates.
For example, the difference in months between 8/31/2012 and 9/1/2012 is 1, eventhough it's only one day between the dates.
If you want to consider the day component you have to get the date difference in days instead of months, and calculate how many months you want that to be.

Answer 3

This is the class I use (it's C# but really easy to convert to VB.NET).
It is usefull for years, months, days... it is ideal for displaying ages in #Y-#M-#D format.
public class DateDifference
{
/// <summary>
/// defining Number of days in month; index 0=> january and 11=> December
/// february contain either 28 or 29 days, that's why here value is -1
/// which wil be calculate later.
/// </summary>
private int[] monthDay = new int[12] { 31, -1, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
/// <summary>
/// contain from date
/// </summary>
private DateTime fromDate;
/// <summary>
/// contain To Date
/// </summary>
private DateTime toDate;
/// <summary>
/// this three variable for output representation..
/// </summary>
private int year;
private int month;
private int day;
public DateDifference(DateTime d1, DateTime d2)
{
int increment;
if (d1 > d2)
{
this.fromDate = d2;
this.toDate = d1;
}
else
{
this.fromDate = d1;
this.toDate = d2;
}
///
/// Day Calculation
///
increment = 0;
if (this.fromDate.Day > this.toDate.Day)
{
increment = this.monthDay[this.fromDate.Month - 1];
}
/// if it is february month
/// if it's to day is less then from day
if (increment == -1)
{
if (DateTime.IsLeapYear(this.fromDate.Year))
{
// leap year february contain 29 days
increment = 29;
}
else
{
increment = 28;
}
}
if (increment != 0)
{
day = (this.toDate.Day + increment) - this.fromDate.Day;
increment = 1;
}
else
{
day = this.toDate.Day - this.fromDate.Day;
}
///
///month calculation
///
if ((this.fromDate.Month + increment) > this.toDate.Month)
{
this.month = (this.toDate.Month + 12) - (this.fromDate.Month + increment);
increment = 1;
}
else
{
this.month = (this.toDate.Month) - (this.fromDate.Month + increment);
increment = 0;
}
///
/// year calculation
///
this.year = this.toDate.Year - (this.fromDate.Year + increment);
}
public override string ToString()
{
//return base.ToString();
return this.year + " Year(s), " + this.month + " month(s), " + this.day + " day(s)";
}
public int Years
{
get
{
return this.year;
}
}
public int Months
{
get
{
return this.month;
}
}
public int Days
{
get
{
return this.day;
}
}
}
USAGE:
DateDifference diff = new DateDifference(date1, date2);
int months = (diff.Years*12) + diff.Months + diff.Days > 15 ? 1 : 0;

Answer 4

If you want the number of complete month, then you'll need to compare date that starts at the begining of the closet month.
Sub Main()
' mnth = DateDiff(DateInterval.Month, 8/30/2012, 10/1/2012)
Console.WriteLine(GetCompleteMonthCount(New DateTime(2012, 8, 30), New DateTime(2012, 10, 1)))
Console.ReadLine()
End Sub
Public Function GetCompleteMonthCount(ByVal d1 As DateTime, ByVal d2 As DateTime) As Integer
If d1.Day <> 1 Then
d1 = d1.AddMonths(1)
d1 = New DateTime(d1.Year, d1.Month, 1)
End If
If d2.Day <> 1 Then
d2 = New DateTime(d2.Year, d2.Month, 1)
End If
Return DateDiff(DateInterval.Month, d1, d2)
End Function

Answer 5

The solutions above are all very good but perhaps a little over-complicated, how about
Function wholeMonthsEd(d1, d2) As Integer
' determine the DateDiff function output
' which gives calendar month difference
initMonths = DateDiff("m", d1, d2)
' do calcs on the Day of the month to deduct/not a calendar month
If Day(d2) < Day(d1) Then
initMonths = initMonths - 1
End If
wholeMonths = initMonths
End Function