automatically working formula

You'll need to use VBA.

See Help for the OnTime method.

Maybe like this.

In ThisWorkbook:

Private Sub Workbook_Open()
    Application.OnTime Date + TimeValue("08:00"), "AddOneEveryFive"
End Sub

In a code module:

Sub AddOneEveryFive()
    With Range("A1")
        If IsNumeric(.Value) Then .Value = .Value + 1
    End With
    
    If Time < TimeValue("15:55") Then
        Application.OnTime Now + TimeValue("00:05"), "AddOneEveryFive"
    End If
End Sub