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How do I calculate due date with whole months?

I'm trying to calculate the due date of an invoice, and I have a date of invoice, days to due date (100) and I need to calculate due date with whole months:

For example:
if date of invoice = 07/05/2009 and days of due date = 120, then the due date calculated = 07/09/2009 (4 months exactly -whole months-).
My code:
DateTime invoiceDate=System.DateTime.Now();
int numberOfDays=100;
int numberOfMonths;
int restOfNumberOfDays;
Date dueDate;
    numberOfMonths = numberOfDays % 30;
    dueDate = dt.AddMonths(numberOfMonths);
    restOfNumberOfDays = numberOfDays - (numberOfMonths * 30);
    dueDate = dueDate.AddDays(restOfNumberOfDays);
This code would be correct if the month had 30 days long.

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4 Solutions
There is a DateDiff method in Visual Basic which returns the value in months.

Just go through this site for details

The decompiled code for DateDiff is provided there...

you need to convert it to C#.
Here is the code...
Public Shared Function DateDiff(ByVal Interval As DateInterval, ByVal Date1 As DateTime, ByVal Date2 As DateTime, ByVal Optional DayOfWeek As FirstDayOfWeek = 1, ByVal Optional WeekOfYear As FirstWeekOfYear = 1) As Long
Dim currentCalendar As Calendar
Dim span As TimeSpan = Date2.Subtract(Date1)
Select Case Interval
Case DateInterval.Year
currentCalendar = DateAndTime.CurrentCalendar
Return CLng((currentCalendar.GetYear(Date2) - currentCalendar.GetYear(Date1)))
Case DateInterval.Quarter
currentCalendar = DateAndTime.CurrentCalendar
Return CLng(((((currentCalendar.GetYear(Date2) - currentCalendar.GetYear(Date1)) * 4) + ((currentCalendar.GetMonth(Date2) - 1) / 3)) - ((currentCalendar.GetMonth(Date1) - 1) / 3)))
Case DateInterval.Month
currentCalendar = DateAndTime.CurrentCalendar
Return CLng(((((currentCalendar.GetYear(Date2) - currentCalendar.GetYear(Date1)) * 12) + currentCalendar.GetMonth(Date2)) - currentCalendar.GetMonth(Date1)))
Case DateInterval.DayOfYear, DateInterval.Day
Return CLng(Math.Round(Conversion.Fix(span.TotalDays)))
Case DateInterval.WeekOfYear
Date1 = Date1.AddDays(CDbl((0 - DateAndTime.GetDayOfWeek(Date1, DayOfWeek))))
Date2 = Date2.AddDays(CDbl((0 - DateAndTime.GetDayOfWeek(Date2, DayOfWeek))))
Return (CLng(Math.Round(Conversion.Fix(Date2.Subtract(Date1).TotalDays))) / 7)
Case DateInterval.Weekday
Return (CLng(Math.Round(Conversion.Fix(span.TotalDays))) / 7)
Case DateInterval.Hour
Return CLng(Math.Round(Conversion.Fix(span.TotalHours)))
Case DateInterval.Minute
Return CLng(Math.Round(Conversion.Fix(span.TotalMinutes)))
Case DateInterval.Second
Return CLng(Math.Round(Conversion.Fix(span.TotalSeconds)))
End Select
Throw New ArgumentException(Utils.GetResourceString("Argument_InvalidValue1", New String() { "Interval" }))
End Function 

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I think this is what they are doing for difference in months

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

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there is an already method to add days to datetime objct in .Net, all what you have to do is add number of days to the method AddDays()

DateTime duedate = DateTime.Now
duedate = duedate.AddDays(120)

Well, if you want every month to be exactly 30 days, you could use the following method:
System.DateTime CalculateDueDate(System.Int32 myDaysUntilDue)
     return System.DateTime.Now.AddMonths(myDaysUntilDue / 30);

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