A worksheet change function like the one below should work. Format sheet 5
column B for time (any time format you prefer).  I placed the code below on
sheet 1 in the VBA project window.  Worksheet change must be placed in every
worksheet you want the action to take place.  If you place the code on sheet
5 then you have to add an if statement to ignore range A1:B7.

Sub worksheet_change(ByVal Target As Range)

Myrow = Weekday(Now() - 1, vbMonday)
If Hour(Now()) < 21 Then
  Sheets("Sheet5").Cells(Myrow, "B") = _
     Now()
End If
End Sub

Put the following code in a standard code module, then call it from a
button on a toolbar or worksheet. It formats the cell's number format
automatically as each value is entered, or you could simply format the
cells in column B ahead of time.

'----------------------------------------------------------------------
Public Sub StampTime()
 Dim dblNow As Double
 Dim dblTimeNow As Double
 Dim lngDayOfWeek As Long
 Dim lngStampRow As Long
 Dim rngStampCell As Range

 dblNow = Now

 With WorksheetFunction
   dblTimeNow = dblNow - .RoundDown(dblNow, 0)
   lngDayOfWeek = .Weekday(.RoundDown(dblNow, 0))
 End With

 lngStampRow = ((lngDayOfWeek + 4) Mod 7) + 1

 Set rngStampCell = Sheet5.Cells(lngStampRow, 2)

 With rngStampCell
   If (dblTimeNow > (21# / 24#)) _
     Then
       .ClearContents
     Else
       .Value = dblNow
       .NumberFormat = "hh:mm"
     End If
 End With
End Sub