What do you mean by "if the Date field doesn't exist yet"? If it is an available field, you can just use:
PT.Pivotfields("Date").Orientation = xlColumnField
which will add it if it's not in the pivot table and won't do anything if it's already a column field. Otherwise, you could use the loop to check for it like this:
Dim pt As PivotTable, pf As PivotField, blnFound As Boolean
Set pt = ActiveSheet.PivotTables(1)
For Each pf In pt.ColumnFields
If pf.Name = "Date" Then
blnFound = True
Exit For
End If
Next pf
If Not blnFound Then
' Date field not present
Else
' Date field already is a column field.
End If