I found this piece of code while searching for a way to underline
anytime there is a date change in column A, but it doesnt always
underline correctly and if ran twice it incorrectly underlines rows.
Where is the bug? I can't find it. Any help is appreciated. thanks!

Dim RowNdx As Long
Dim ColNum As Integer

ColNum = Selection(1).Column
Columns("A:A").Select
Application.ScreenUpdating = False

For RowNdx = Selection(Selection.Cells.Count).Row To _
Selection(1).Row + 1 Step -1
If Cells(RowNdx, ColNum).Value < Cells(RowNdx - 1, ColNum).Value
Then
Cells(RowNdx - 1, 1).EntireRow.Select
With Selection.Borders(xlEdgeBottom)
.LineStyle = xlContinuous
.Weight = xlMedium
.ColorIndex = xlAutomatic
End With
End If
Next RowNdx