Before running the macro below, select the dates, a single row with dates in every cell. This selection is what the macro will look at.
Make sure when you run it that the active sheet is the one you want to add columns to.
Set Rng = Selection
For c = Rng.Columns.Count To 2 Step -1
If Application.EoMonth(Rng.Cells(c).Value, 0) <> Application.EoMonth(Rng.Cells(c - 1).Value, 0) Then