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How do I calculate due date with whole months?

Posted on 2009-05-07
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Last Modified: 2013-12-17
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;
 
if(wholeMonths)
{
    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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Question by:ReaL10
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4 Comments
 
LVL 16

Accepted Solution

by:
CuteBug earned 300 total points
ID: 24324354
There is a DateDiff method in Visual Basic which returns the value in months.

Just go through this site for details
http://www.csharpfriends.com/Forums/ShowPost.aspx?PostID=52580

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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LVL 16

Assisted Solution

by:CuteBug
CuteBug earned 300 total points
ID: 24324366
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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LVL 4

Assisted Solution

by:cauos
cauos earned 100 total points
ID: 24324389
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)

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LVL 9

Assisted Solution

by:tculler
tculler earned 100 total points
ID: 24325458
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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