VB.NET : Populating Gridview with Dates (30 days interval) - vb.net

Question

I am trying to populate a gridview with dates.
For example the date today is 2/18/2019 and the next date within 30 days is 3/20/2019. so the desired output shoul'd look like this
No. Date
1 3/20/2019
2 4/19/2019
3 5/19/2019
etc.
but the result is
No. Date
1 3/20/2019
2 3/20/2019
3 3/20/2019
etc.
Here is my code so far.
Dim termCounter As Integer = 36
Dim today As DateTime = DateTime.Today
Dim dueDate As DateTime = today.AddDays(30)
Dim dtable As DataTable = New DataTable()
dtable.Columns.Add("No.")
dtable.Columns.Add("Payment Date")
Dim RowValues As Object() = {"", ""}
Dim dRow As DataRow
Dim tmpDate As Date
For i As Integer = 1 To termCounter
If GridAmortSched.RowCount <= 0 Then
RowValues(0) = i
RowValues(1) = dueDate.ToShortDateString
dRow = dtable.Rows.Add(RowValues)
Else
tmpDate = GridAmortSched.Rows(GridAmortSched.RowCount - 1).Cells(1).Value.ToString()
RowValues(0) = i
RowValues(1) = tmpDate.AddDays(30).ToShortDateString
dRow = dtable.Rows.Add(RowValues)
End If
Next
dtable.AcceptChanges()
GridAmortSched.DataSource = dtable

Answer 1

As is often the case, the code is behaving as you have told it to do, not at you want it to do :-).
Every time you go through the loop, you set tmpDate from the same source and then two lines later, you put the value into RowValues. :
tmpDate = GridAmortSched.Rows(GridAmortSched.RowCount - 1).Cells(1).Value.ToString()
You do not adjust your source by i, nor do you increment it. When you AddDays, you do not adjust the original value either (which would not work because of how and when you assign it).
Two easy fixes (two options)
One. The first is to index the days to be added against the loop value - the simple modification to the code below will achieve this. It is a quick fix and may be harder to maintain in the future - more to the point, is harder to spot if you change something such as the loop starting point.
RowValues(1) = tmpDate.AddDays(30*i).ToShortDateString
Two. The other option is to address the code logic. Set your baseline value and then increment it within the loop.
Dim tmpDate As Date
tmpDate = GridAmortSched.Rows(GridAmortSched.RowCount - 1).Cells(1).Value.ToString() '*** Moved outside of loop
For i As Integer = 1 To termCounter
If GridAmortSched.RowCount <= 0 Then
RowValues(0) = i
RowValues(1) = dueDate.ToShortDateString
dRow = dtable.Rows.Add(RowValues)
Else
RowValues(0) = i
tmpDate = tmpDate.AddDays(30) '*** Increment date every time it passes here
RowValues(1) = tmpDate.ToShortDateString '*** Assign the new value
dRow = dtable.Rows.Add(RowValues)
End If
Next
dtable.AcceptChanges()
GridAmortSched.DataSource = dtable