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get the first date of a particular day for every month

Posted on 2011-02-11
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Last Modified: 2012-05-11
I have a date , based on that date I need to find the first date of that particualr day for a date range

So if I have a start date and end date. And my date is say feb 7th, which is a monday, then I need to find all the first mondays for every month between the start and end date.
Does not matter if it is a holiday.
So my next day will be Mrch 8th, Apr 4th and son unitl I reach the end date boundary
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Question by:countrymeister
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4 Comments
 
LVL 75

Accepted Solution

by:
käµfm³d   👽 earned 2000 total points
ID: 34872268
Something like this?

Logic
Public Function GetFirstDays(ByVal startDate As DateTime, ByVal endDate As DateTime) As List(Of DateTime)
	Dim result As New List(Of DateTime)
	Dim day As DayOfWeek = startDate.DayOfWeek
	Dim temp As New DateTime(startDate.Year, startDate.Month, 1)

	temp = temp.AddMonths(1)

	While temp < endDate
		While temp.DayOfWeek <> day
			temp = temp.AddDays(1)
		End While

		If temp < endDate Then result.Add(temp)

		temp = New DateTime(temp.Year, temp.Month, 1)
		temp = temp.AddMonths(1)
	End While

	Return result
End Function

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Usage
Sub Main()
    Dim start As New DateTime(2011, 2, 7)
    Dim halt As New DateTime(2011, 5, 1)
    Dim days As List(Of DateTime) = GetFirstDays(start, halt)
End Sub

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LVL 83

Expert Comment

by:CodeCruiser
ID: 34872295
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LVL 86

Expert Comment

by:Mike Tomlinson
ID: 34872649
*This is very similar to kaufmed's answer...it seemed to me that the reference date might not actually be the start date (or even within the range):
Private Function FirstDays(ByVal startDT As DateTime, ByVal stopDT As DateTime, ByVal refDT As DateTime) As List(Of DateTime)
        Dim results As New List(Of DateTime)
        Dim targetDay As DayOfWeek = refDT.DayOfWeek

        Dim tmpDT As DateTime
        Dim dt As New DateTime(startDT.Year, startDT.Month, 1)
        While dt <= stopDT
            tmpDT = dt
            While tmpDT.DayOfWeek <> targetDay
                tmpDT = tmpDT.AddDays(1)
            End While
            If tmpDT >= startDT AndAlso tmpDT <= stopDT Then
                results.Add(tmpDT)
            End If
            dt = dt.AddMonths(1)
        End While

        Return results
    End Function

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LVL 63

Expert Comment

by:Fernando Soto
ID: 34873431
Hi countrymeister;

If I understand the question correctly this should do what you want.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim dts As DateTime = New DateTime(2011, 11, 12)
    Dim dte As DateTime = New DateTime(2012, 2, 20)
    Dim dates As List(Of DateTime)
    dates = GetMonthStarts(dts, dte)

End Sub

Private Function GetMonthStarts(ByVal startDate As DateTime, ByVal endDate As DateTime) As List(Of DateTime)

    ' Used to return the list of DateTime
    Dim dates As New List(Of DateTime)
    ' Get the day of the week to use as the first day of the month
    Dim startDay As Integer = startDate.DayOfWeek
    ' Used to keep track of whivh month we are working on
    Dim processDate As DateTime = startDate.AddMonths(1)
    ' Starting with month
    Dim startMonth As Integer = startDate.Month
    ' Total number of months in the range
    Dim totalMonths As Long = DateDiff(DateInterval.Month, startDate, endDate)

    ' Add the start date to the list to be returned
    dates.Add(startDate)

    For idx As Integer = 0 To totalMonths - 2
        ' Used to see which day of the begining of the month we want to get
        Dim calcDate As DateTime = New DateTime(processDate.Year, processDate.Month, 1)

        ' See if the calcDate is the one we are looking for
        If calcDate.DayOfWeek = startDay Then
            ' Add to the list of returned dates
            dates.Add(calcDate)
            ' get ready for the next month
            processDate = processDate.AddMonths(1)
        Else
            ' Find the day in the begining of the month we are looking for.
            Do
                If calcDate.DayOfWeek = startDay Then
                    dates.Add(calcDate)
                    processDate = processDate.AddMonths(1)
                    Exit Do
                End If
                calcDate = calcDate.AddDays(1)
            Loop While calcDate.DayOfWeek <= startDay
        End If
    Next

    Return dates

End Function

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Fernando
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