Use the Get Date/Time String VI (On the Time & Dialogue palette). Wire the time string output of this to the Write Characters To File VI on the File IO palette. Use the same file path as your speadsheet. (I am assuming you are using the Write to Speadsheet File VI). Make sure you have append to file set to true, so it adds the date / time to the end of your data and does not overwrite it.
Nick.