I can track down the problem  caused by
extensive recalculation whenever a sheet  is deleted.

Therefore, I have to turn off the automatic recalculation and turn it
on again  at the end.

Sub DeleteOld()

Application.Calculation = xlCalculationManual

For Each Sheet In Worksheets

If IsDate(Sheet.Name) Then
If DateValue(Sheet.Name) <= DateValue(Date - 14) Then
Application.DisplayAlerts = False
Sheet.Delete
End If

End If
Next

Application.Calculation = xlCalculationAutomatic

End Sub