Rolling Sum in VBA - excel

Question

I am trying to create a rolling twelve month sum in excel using vb. The last column with data is dynamic so I created a variable to store that column number each time. I then tried to create a loop that would output the sum of the previous 12 cells and move to the next cell until it reached 12 calculation cells. But this is not working... Is there something obvious that I am doing wrong?? Thanks!
Sub OSR_ReportComplete()
Dim lCol As Long
Dim p As Integer
lCol = Cells(7, Columns.Count).End(xlToLeft).Column
For p = 0 To 12
Range(Cells(15, lCol - p)).Value = Application.Sum(Range(Cells(7, lCol -
p), Cells(7, lCol - p - 12)))
Next p
End Sub

Answer 1

Could be done in a neater way but this works
Sub OSR_ReportComplete()
Dim lCol As Long
Dim p As Integer
lCol = Cells(7, Columns.Count).End(xlToLeft).Column
For p = 0 To 12
Cells(15, lCol - p) = Application.Sum(Range(Cells(7, lCol - p), Cells(7, lCol - p - 12)))
Next p
End Sub
Also, If your lCol evaluates to a value less then 25 it will fall over though due to trying to reference a column less than 1. I'd recommend using Offset instead
However, a rolling total can be done very easily in excel formulas also. Say your first value is in Column A and your date is in row 1, in row 2 enter in Column A =SUM($A$1:A1) and drag along the rest of your range
Giving:
Formulas:
The following code will generate the formulas above for row 1 in one line instead of doing your loop. This is an example as I don't understand the layout of your sheet from your code
Sub OSR_ReportComplete2()
With Range(Cells(1, 1), Cells(1, Columns.Count).End(xlToLeft))
.Offset(1, 0).Formula = "=SUM(" & .Cells(1).Address & ":" & Split(.Cells.Address(RowAbsolute:=False, ColumnAbsolute:=False), ":")(0) & ")"
End With
End Sub
This is easier to write in R1C1 notation (however will be displayed in A1 in Excel Sheet)
Sub OSR_ReportComplete2()
With Range(Cells(1, 1), Cells(1, Columns.Count).End(xlToLeft))
.Offset(1, 0).FormulaR1C1 = "=SUM(" & .Cells(1).Address(ReferenceStyle:=False) & ":R[-1]C)"
End With
End Sub

Answer 2

First of all, I suggest you don't use VBA for this. Instead, use the inbuilt OFFSET function, like this.
Your code is quite complex, but maybe you should be using xlToRight instead of xlToLeft to find the last column.
If you want to use VBA, you can try using WorkSheetFunction.offset, putting it into a range variable and then summing that range.