Formula in a macro doesnt work for Spanish Excel user

I created the loop:

Sub DateMod()
    Dim Temp    As Variant, _
        Prefix  As Variant, _
        Yr      As Variant, _
        Mnth    As String, _
        DayNum  As String, _
        DV      As Date
        
       Dim lastColumn As Long
       Dim i As Integer
       Dim sht As Worksheet
       
Set sht = ThisWorkbook.Worksheets("Forecast")
Range("D1").Select
 lastColumn = sht.Cells(1, sht.Columns.Count).End(xlToLeft).Column

 
 MsgBox lastColumn
  
    For i = 4 To lastColumn
    
    
    Temp = Split(Cells(1, i).Value, "\")
    Yr = Split(Temp(0), "-")
    Prefix = Yr(0)
    Yr = Yr(1)
    Temp = Split(Temp(1), " ")
    Mnth = Temp(1)
    DayNum = Left(Temp(0), Len(Temp(0)) - 2)
    DV = DateValue(DayNum & " " & Mnth & " " & Yr)
    Cells(2, i).Value = DV
    Next i
End Sub

This worked great for me but the user got a type mismatch this is what her screen looked like after running:

10-30-2018 3-12-49 PM.jpg
Rows 3 and 4 were not in date format previously.