Retest timing

Is the 10 day retest time-stamped according to time of day of the original test, or does it run in whole days? Usually, our customers are very good and don't leave coming back to the last minute so it's not a problem but would be good to know. 
A while ago we did one on what we thought was the 10th day and it wasn't allowed by the computer - said the 10 days were up - but not seen the issue again, however would like to be sure on exactly how it works!