I likewise use a checkmark to indicate a work is finished, but I also include a time stamp many columns over. By clicking on a cell in Column B or C, my modified script inserts an "a" using a Marlett font, which creates a checkmark if before 10 am, otherwise, an "r" creates an X.
Perhaps this can be changed to fit your needs by adding a new section for 17:30. Since VBA is "dynamic" on cell changes, I prefer using it over a formula (just clicking in the cell). The check mark or X is removed by clicking in and out of the same cell once again.
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
'Mark Task completed if finished before 10 am
If Target.Cells.Count > 1 Then Exit Sub
'Only looking at column B & C, rows 3 thru 31
If Not Intersect(Target, Range("B3:C31")) Is Nothing Then
Target.Font.Name = "Marlett"
If Target = vbNullString Then
If Time() < "10:00:00" Then
Target = "a"
Target = "r
Target = vbNullString
Hope this helps you.