calculate date add, but only weekdays - vb.net

Question

I would like to calculate a new date simply by using the build-in dateadd function, but take into account that only weekdays should be counted (or 'business days' so to speak).
I have come up with this simple algorithm, which does not bother about holidays and such. I have tested this with some simple dates, but would like some input if this can be done in better ways.
This sample assumes a week with 5 business days, monday-friday, where first day of the week is monday. Dateformatting used here is d-m-yyyy, the sample calculates with a startdate of october 1, 2009.
Here is the simple form:
Dim d_StartDate As DateTime = "1-10-2009"
Dim i_NumberOfDays As Integer = 12
Dim i_CalculateNumberOfDays As Integer
If i_NumberOfDays > (5 - d_StartDate.DayOfWeek) Then
i_CalculateNumberOfDays = i_NumberOfDays
Else
i_CalculateNumberOfDays = i_NumberOfDays + (Int(((i_NumberOfDays + (7 - d_StartDate.DayOfWeek)) / 5)) * 2)
End If
MsgBox(DateAdd(DateInterval.Day, i_CalculateNumberOfDays, d_StartDate))
Which I try to explain with the following piece of code:
''' create variables to begin with
Dim d_StartDate as Date = "1-10-2009"
Dim i_NumberOfDays as Integer = 5
''' create var to store number of days to calculate with
Dim i_AddNumberOfDays as Integer
''' check if amount of businessdays to add exceeds the
''' amount of businessdays left in the week of the startdate
If i_NumberOfDays > (5 - d_StartDate.DayOfWeek) Then
''' start by substracting days in week with the current day,
''' to calculate the remainder of days left in the current week
i_AddNumberOfDays = 7 - d_StartDate.DayOfWeek
''' add the remainder of days in this week to the total
''' number of days we have to add to the date
i_AddNumberOfDays += i_NumberOfDays
''' divide by 5, because we need to know how many
''' business weeks we are dealing with
i_AddNumberOfDays = i_AddNumberOfDays / 5
''' multiply the integer of current business weeks by 2
''' those are the amount of days in the weekends we have
''' to add to the total
i_AddNumberOfDays = Int(i_AddNumberOfDays) * 2
''' add the number of days to the weekend days
i_AddNumberOfDays += i_NumberOfDays
Else
''' there are enough businessdays left in this week
''' to add the given amount of days
i_AddNumberOfDays = i_NumberOfDays
End If
''' this is the numberof dates to calculate with in DateAdd
dim d_CalculatedDate as Date
d_CalculatedDate = DateAdd(DateInterval.Day, i_AddNumberOfDays, d_StartDate)
Thanks in advance for your comments and input on this.

Answer 1

I used the .DayOfWeek function to check if it was a weekend. This does not include holiday implementation. It has been tested. I realize this question is old but the accepted answer didn't work. However, I did like how clean it was so I thought I'd update it and post. I did change the logic in the while loop.
Function AddBusinessDays(startDate As Date, numberOfDays As Integer) As Date
Dim newDate As Date = startDate
While numberOfDays > 0
newDate = newDate.AddDays(1)
If newDate.DayOfWeek() > 0 AndAlso newDate.DayOfWeek() < 6 Then '1-5 is Mon-Fri
numberOfDays -= 1
End If
End While
Return newDate
End Function

Answer 2

Public Shared Function AddBusinessDays(ByVal startDate As DateTime, _
ByVal businessDays As Integer) As DateTime
Dim di As Integer
Dim calendarDays As Integer
'''di: shift to Friday. If it's Sat or Sun don't shift'
di = (businessDays - Math.Max(0, (5 - startDate.DayOfWeek)))
''' Start = Friday -> add di/5 weeks -> end = Friday'
''' -> if the the remaining (<5 days) is > 0: add it + 2 days (Sat+Sun)'
''' -> shift back to initial day'
calendarDays = ((((di / 5) * 7) _
+ IIf(((di Mod 5) <> 0), (2 + (di Mod 5)), 0)) _
+ (5 - startDate.DayOfWeek))
Return startDate.AddDays(CDbl(calendarDays))
End Function

Answer 3

Your plan seems like it should work. Make sure you wrap it in a function instead of doing out the calculations every place you use it so that if/when you discover you need to account for holidays, you don't have to change it in tons of places.
The best way I can think of for implementing support for holidays would be to add days one at a time in a loop. Each iteration, check if its a weekend or a holiday, and if it is add another day and continue (to skip it). Below is an example in pseudocode (I don't know VB); no guarantees its correct. Of course, you need to provide your own implementations for isWeekend() and isHoliday().
function addBusinessDays(startDate, numDays)
{
Date newDate = startDate;
while (numDays > 0)
{
newDate.addDays(1);
if (newDate.isWeekend() || newDate.isHoliday())
continue;
numDays -= 1;
}
return newDate;
}
My first thought for the holiday thing was to simply look up the number of holidays between the start date and the end date and add that to your calculation, but of course this won't work because the end date is dependent on the number of holidays in that interval. I think an iterative solution is the best you'll get for holidays.

Answer 4

I'm using this code to calculate the date:
dayOfWeek = startDate.DayOfWeek
weekendDays = 2 * Math.Floor((dayOfWeek + businessDays - 0.1) / 5)
newDate = startDate.AddDays(businessDays + weekendDays)
The second line computes the number of full weekends that we have to add and then multiplies them by 2 to obtain the number of days.
The additional -0.1 constant is used to avoid adding days if (dayOfWeek + businessDays) is multiple of 5, and final date is friday.

Answer 5

Private Function AddBusinessDays(ByVal dtStartDate As DateTime, ByVal intVal As Integer) As DateTime
Dim dtTemp As DateTime = dtStartDate
dtTemp = dtStartDate.AddDays(intVal)
Select Case dtTemp.DayOfWeek
Case 0, 6
dtTemp = dtTemp.AddDays(2)
End Select
AddBusinessDays = dtTemp
End Function

Answer 6

please check this code for adding working days
Dim strDate As Date = DateTime.Now.Date
Dim afterAddDays As Date
Dim strResultDate As String
Dim n As Integer = 0
For i = 1 To 15
afterAddDays = strDate.AddDays(i)
If afterAddDays.DayOfWeek = DayOfWeek.Saturday Or afterAddDays.DayOfWeek = DayOfWeek.Sunday Then
n = n + 1
End If
Next
strResultDate = afterAddDays.AddDays(n).ToShortDateString()

Answer 7

Private Function AddXWorkingDays(noOfWorkingDaysToAdd As Integer) As Date
AddXWorkingDays = DateAdd(DateInterval.Weekday, noOfWorkingDaysToAdd + 2, Date.Today)
If Weekday(Today) + noOfWorkingDaysToAdd < 6 Then AddXWorkingDays = DateAdd(DateInterval.Weekday, 2, Date.Today)
End Function

Answer 8

One approach would be to iterate from the start date and add or subtract one day with each iteration if the date is not a Saturday or Sunday. If zero is passed as iAddDays then the function will return dDate even if that date is a Saturday or Sunday. You could play around with the logic to get the outcome you are looking for if that scenario is a possibility.
Public Function DateAddWeekDaysOnly(ByVal dDate As DateTime, ByVal iAddDays As Int32) As DateTime
If iAddDays <> 0 Then
Dim iIncrement As Int32 = If(iAddDays > 0, 1, -1)
Dim iCounter As Int32
Do
dDate = dDate.AddDays(iIncrement)
If dDate.DayOfWeek <> DayOfWeek.Saturday AndAlso dDate.DayOfWeek <> DayOfWeek.Sunday Then iCounter += iIncrement
Loop Until iCounter = iAddDays
End If
Return dDate
End Function

Answer 9

Easy way
function addWerktage($date,$tage){
for($t=0;$t<$tage;$t++){
$date = $date + (60*60*24);
if(date("w",$date) == 0 || date("w",$date) == 6){ $t--; }
}
return $date;
}

Answer 10

This produces the same result as the accepted answer, including starting on weekends, while handling negative offsets and without looping. It's written in c# but should work in any enviroment where numeric weekdays start at Sunday and end at Saturday, and where integer division rounds to 0.
public static DateTime AddWeekdays(DateTime date, int offset)
{
if (offset == 0)
{
return date;
}
// Used to calculate the number of weekend days skipped over
int weekends;
if (offset > 0)
{
if (date.DayOfWeek == DayOfWeek.Saturday)
{
// Monday is 1 weekday away, so it will take an extra day to reach the next weekend
int daysSinceMonday = -1;
// Add two weekends for every five days
int normalWeekends = (offset + daysSinceMonday) / 5 * 2;
// Add one for this Sunday
int partialWeekend = 1;
weekends = normalWeekends + partialWeekend;
}
else
{
// It will take this many fewer days to reach the next weekend.
// Note that this works for Sunday as well (offset -1, same as above)
int daysSinceMonday = date.DayOfWeek - DayOfWeek.Monday;
// Add two weekends for every five days (1 business week)
weekends = (offset + daysSinceMonday) / 5 * 2;
}
}
else
{
// Same as the positive offset case, but counting backwards.
// daysSinceFriday will be 0 or negative, except for Saturday, which is +1
int daysSinceFriday = date.DayOfWeek - DayOfWeek.Friday;
weekends = date.DayOfWeek == DayOfWeek.Sunday
? (offset + 1) / 5 * 2 - 1
: (offset + daysSinceFriday) / 5 * 2;
}
return date.AddDays(offset + weekends);
}
Since the pattern of 2 extra days per 5 days repeats after a full week, you can effectively exhaustively test it:
private static DateTime AddWeekdaysLooping(DateTime date, int offset)
{
DateTime newDate = date;
int step = Math.Sign(offset);
while (offset != 0)
{
newDate = newDate.AddDays(step);
if (newDate.DayOfWeek != DayOfWeek.Sunday && newDate.DayOfWeek != DayOfWeek.Saturday)
{
offset -= step;
}
}
return newDate;
}
void TestWeekdays()
{
DateTime initial = new DateTime(2001, 1, 1);
for (int day = 0; day < 7; day += 1)
{
for (int offset = -25; offset <= 25; offset += 1)
{
DateTime start = initial.AddDays(day);
DateTime expected = AddWeekdaysLooping(start, offset);
DateTime actual = AddWeekdays(start, offset);
if (expected != actual) {
throw new Exception($"{start.DayOfWeek} + {offset}: expected {expected:d}, but got {actual:d}");
}
}
}
}

Answer 11

Dim result As Date
result = DateAdd("d", 2, Today)
If result.DayOfWeek = DayOfWeek.Saturday Then
result = DateAdd("d", 2, result)
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Sunday Then
result = DateAdd("d", 1, result)
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Monday Then
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Tuesday Then
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Wednesday Then
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Thursday Then
MsgBox(result)
ElseIf result.DayOfWeek = DayOfWeek.Friday Then
MsgBox(result)
End If