4.6 Methods for handling time

We decided to represent the time of day with a single slot named total-seconds. An alternate choice would be to give the class three slots, named hours, minutes, and seconds. People naturally think of time in terms of hours, minutes, and seconds. We chose to store the total seconds instead, because we envisioned needing to operate on times, such as adding a time of day to a time offset. For example, if it is 9:00 now, and a meeting is to be held 2.5 hours from now, then the meeting will be held at 11:30. It is easier to operate on a single value, rather than on three values of hours, minutes, and seconds. On the other hand, it is convenient to see times expressed as hours, minutes, and seconds. We can represent the instances with a single slot, and can provide methods that let users create and see <time-of-day> instances as being hours, minutes, and seconds.