Auto Populating a cell at a specific time if nothing has been entered in the cell.

I have a sheet that associates enter their parts per hour on. This is connected to a graph that updates every 15 min. Sometimes there is no operator at a machine, and if someone doesn't physically enter No Operator into the sheet, then the graph is blank. I would like a macro that would check to see if anything has been entered into the cell 15 minutes after every hour. If nothing is in the Cell, I would like the Cell to Auto Populate "No Operator".

Thanks.