Extremely slow VBA code when formatting cells - vba

Question

When attempting to format 2 columns in a workbook, the execution of the macro is extremely slow. To format approximately 4000 rows, it takes over 10 minutes.
The dates are populated from an external source that stores them as strings.
When commenting the code, it loads under 60 seconds.
The code
'Discover last row of data
RowsToProcess = Range("A" & Rows.Count).End(xlUp).Row
For i = 6 To RowsToProcess
Worksheets("Data").Range("B" & i).Select
Selection.NumberFormat = "dd/mm/yy;;"
Selection.Value = CDate(Selection.Value)
Worksheets("Data").Range("C" & i).Select
Selection.NumberFormat = "dd/mm/yy;;"
Selection.Value = CDate(Selection.Value)
Next i
The code below does not format cells in the required format either.
Worksheets("Data).Columns("C").NumberFormat = dd/mm/yy;;"

Answer 1

The post #aelgoa linked to is spot on. When the standard Application.ScreenUpdating options for speeding up your code aren't enough, I turn to Variant arrays.
(If you wanted to see how I use Application.ScreenUpdating etc., wrapped in a GoFast function, check out my answer here: VBA code optimization)
The script below works like this:
Load the Range defined in columns B and C into a Variant array
Apply CDate logic there (rather than accessing the Sheet every time)
Write the CDate-modified array out to Sheet
One caveat though -- my question in the comment above about differentiating between mm/dd and dd/mm (say May 6th, 2014 vs June 5th, 2014) still stands. I'll modify the code below based on your thoughts there. Thanks!
Option Explicit
Sub ProcessDates()
Dim AryColBandC As Variant
Dim DateFormatB As Date, DateFormatC As Date
Dim RngColBandC As Range
Dim LastRow As Long, Counter As Long
Dim MySheet As Worksheet
'set references up-front
Set MySheet = ThisWorkbook.Worksheets("Sheet1")
With MySheet
LastRow = .Range("A" & .Rows.Count).End(xlUp).Row
Set RngColBandC = .Range(.Cells(6, 2), .Cells(LastRow, 3))
End With
'load the B-C column range into a variant array
AryColBandC = RngColBandC
'loop through the variant array, applying the date
'conversion to each entry in the array and writing back
For Counter = LBound(AryColBandC) To UBound(AryColBandC)
DateFormatB = CDate(AryColBandC(Counter, 1)) '<~ temporarily store
DateFormatC = CDate(AryColBandC(Counter, 2)) '<~ dates here
AryColBandC(Counter, 1) = DateFormatB
AryColBandC(Counter, 2) = DateFormatC
Next Counter
'write the results out to the sheet
For Counter = LBound(AryColBandC) To UBound(AryColBandC)
MySheet.Cells(5 + Counter, 2) = AryColBandC(Counter, 1)
MySheet.Cells(5 + Counter, 3) = AryColBandC(Counter, 2)
Next Counter
End Sub