It has two sorts of readings: when you scan, but also every 15 mins of its own accord. (This 'historic' reading is actually made up of the average of lots of micro measurements, which means it's more reliable than the scan measurements. I use the historic readings for my calculations, not the scans.)
So just to be clear, even if you only scan once every 8 hrs to transfer the data from the sensor to the reader, all the data will sit and stay on your reader for a matter of months. So when you plug the reader into your PC and export the data, all the historic data - all the every-15-minute numbers - will be there.
That's what I find the most useful. I don't scan that often unless something is up. I look at the numbers later, after a specific meal or un-clever mistake I made.
So yes, you can do a basal test any, any time, just by skipping a meal for the five-hour period. It's WONDERFUL !!