Actually i use excel 2000. so haven't seen your file
but try this.
first, activate the sheet in which this macro will work
Sub DtCols()
Dim rCurrMon As Range
Dim iMonth As Integer
'there should be a date in the cell.
Set rCurrMon = Range("M1") 'or whichever is the cell with the current month in your file
iMonth = Month(rCurrMon)
'startDelete
'there should be dates in the 3 prreceeding cells, in the same row
Dim i As Double
For i = 1 To 3
If Month(rCurrMon) - Month(rCurrMon.Offset(0, -1)) <= 3 Then
Columns(rCurrMon.Offset(0, -1).Column).Delete
End If
Next
'start insert date
Dim dNewDt As Date
For i = 1 To 3
dNewDt = DateAdd("m", i, rCurrMon)
rCurrMon.Offset(0, i) = dNewDt
Next
End Sub