Weird Date change when copying a Worksheet into a new workbook

When I use the following code to create a new workbook with a copied sheet there is a strange date change that occurs:

Worksheets("Week Template").Copy
Set Newwb = ActiveWorkbook
NewwbName = ActiveWorkbook.Name

Windows("Mainworkbook.xlsm").Activate
Sheets("Week Template").Select
    Range("B3:B13").Select
    Application.CutCopyMode = False
    Selection.Copy

Workbooks(NewwbName).Worksheets("Week Template").Activate
Range("B3").Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False

For example, if I copied the date "1/1/2023" it is changed to "12/31/2018"

however, when I change the format of the cells to text, the numbers are the same, "43465"

Does anyone know how I can resolve this issue?

The difference is due to the fact that one of your workbooks (the one with the later date) is set to use the 1904 Date system.

1904 date system was generally used by default on the MAC, and 1900 date system in Windows.

This can be set under File/Excel Options/Advanced/When calculating this workbook:

You can use the Workbook.Date1904 property to determine the workbook setting, and make the appropriate adjustment in the value to keep the dates the same.

Following on from my comment above. Try like this:

    Dim wb As Workbook, wbNew As Workbook, wsTempl As Worksheet
    
    Set wb = ThisWorkbook 'for example, if it's the workbook where this code runs
    Set wsTempl = wb.Worksheets("Week Template")
    
    wsTempl.Copy
    Set wbNew = ActiveWorkbook 'has the copied sheet
    wbNew.Worksheets(wsTempl.Name).Range("B3:B13").Value = wsTempl.Range("B3:B13").Value

If you still see the problem then there's something going on we don't have enough information to figure out. How many dates are affected, what are the date formats applied in the source worksheet, etc.