Originally Posted by Dave Peterson

In your original post, you used a format of dd/mm/yyyy. In this example, you
lost the leading 0's in the day and only used two digit years.

This will work if you really use a format of dd/mm/yyyy. (But remember, you
have to be formatting values--not formulas--formulas can't use this kind of
character by character formatting.)

Option Explicit
Sub testme()
Dim myRng As Range
Dim myCell As Range
Dim iCtr As Long

Set myRng = Nothing
On Error Resume Next
Set myRng = Intersect(Selection, _
Selection.Cells.SpecialCells(xlCellTypeConstants, xlTextValues))

On Error GoTo 0

If myRng Is Nothing Then
MsgBox "No constants"
Exit Sub
End If

For Each myCell In myRng.Cells
For iCtr = 1 To Len(myCell.Value)
If Mid(myCell.Value, iCtr, 10) Like "##/##/####" Then
With myCell.Characters(Start:=iCtr, Length:=10).Font
.Name = "Arial"
.Bold = True
.Size = 12
End With
End If
Next iCtr
Next myCell
End Sub
Dave Peterson