How about an example?
Click on the "Time" button and then enter 67, 55 and 48 in the minutes column then enter 72, 99 and 38 in the seconds column.
Click "C A L C U L A T E" and look at the totals. Notice you don't see 170 for the minute total nor 209 for the seconds total (even though that is what they equal).
In this example, you will never see a minute total nor a second total greater than 59. What happens? Well, 209 seconds are actually 3 minutes and 29 seconds so the calculator just puts 29 in the seconds column and "carries" 3 into the minutes column.
We have a total of 173 for the minutes column, which is actually (2 hours and 53 minutes), so we see 53 as the minute total and it carries the 2 hours into the next column.
Pretty neat huh?