Option Explicit

Sub SaveIt()
Dim xWb As Workbook
Dim dt As String

Application.DisplayAlerts = False

dt = Format(CStr(Now), "yyyy_mm_dd_hh_mm_ss")
On Error Resume Next

For Each xWb In Application.Workbooks
If xWb.Path = vbNullString And Not xWb.ReadOnly And Windows(xWb.Name).Visible Then
    xWb.SaveAs Filename:="C:\Users\Desktop\Excels\Saved" & "\" & dt & " - " & xWb.Name & ".xlsx"
Else
If Not xWb.ReadOnly And Windows(xWb.Name).Visible Then
    xWb.SaveCopyAs Filename:="C:\Users\Desktop\Excels" & "\" & dt & " - " & xWb.Name
End If
End If
Next

Application.DisplayAlerts = True
Application.OnTime Now + TimeValue("00:00:10"), "SaveIt"
End Sub

Sub Workbook_Open()
    Call SaveIt
End Sub

Option Explicit

Sub SaveIt()

Dim xWb As Workbook
Dim dt As String

Application.DisplayAlerts = False

dt = Format(CStr(Now), "yyyy_mm_dd_hh_mm_ss")

For Each xWb In Application.Workbooks

If xWb.Path = vbNullString And Not xWb.ReadOnly And Windows(xWb.Name).Visible Then
    xWb.SaveCopyAs Filename:="C:\User\Desktop\Excels" & "\" & dt & " - " & xWb.Name & ".xlsx"   ' __ ВОТ ЗДЕСЬ ОШИБКА __
Else
If Not xWb.ReadOnly And Windows(xWb.Name).Visible Then
    xWb.SaveCopyAs Filename:="C:\User\Desktop\Excels" & "\" & dt & " - " & xWb.Name
End If

End If

Next

Application.DisplayAlerts = True

Application.OnTime Now + TimeValue("00:05:00"), "SaveIt"

End Sub

Sub test()
Dim c As Range, sh As Worksheet, rRange As Range
For Each sh In ThisWorkbook.Worksheets
On Error Resume Next
Set rRange = sh.Cells.SpecialCells(xlCellTypeFormulas, 23)
    If Err.Number = 0 Then
     For Each c In rRange
      If c.HasFormula Then
          If InStr(1, c.Formula, "SUM") <> 0 Then c.Value = c.Value
      End If
     Next
    Else
     Err.Clear
    End If
Set rRange = Nothing
Next
End Sub

Sub test() 
Dim c As Range 
For Each c In ActiveSheet.UsedRange.Cells 
If c.HasFormula Then
If InStr(1, c.Formula, "SUM") <> 0 Then c.Value = c.Value 
End If
Next
End Sub

Sub test() 
Dim c As Range 
For Each c In ActiveSheet.UsedRange.Cells 
If c.HasFormula Then 
If InStr(1, c.Formula, "SUM"   <> 0 Then c.Value = c.Value 
End If 
Next 
End Sub