Time, Timetables and Time Zones

To turn twenty-four-hour time into twelve-hour, take twelve off any hour of thirteen or more — eighteen fifty is ten to seven in the evening. Minutes roll at sixty, so eighteen fifty plus a quarter of an hour is nineteen oh five, never eighteen sixty-five. Do the arithmetic in twenty-four-hour time and convert at the very end, because there is no am or pm to keep track of while you work.

Crossing midnight, treat it as a landmark: hop to it, note what is left of the duration, then carry on from zero. From twenty-two twenty-five, thirty-five minutes reach twenty-three hundred, an hour more reaches midnight, and fifteen minutes past that give twelve fifteen. The thing people forget is that the date has changed too, so write the day beside the time whenever you cross midnight.

For a journey of more than a few hours, count to midnight first. Leaving Friday at thirteen hundred, eleven hours reach Saturday midnight and the remaining nine give Saturday oh nine hundred. For longer trips, take off twenty-four hours per whole day before anything else — fifty hours is two days and two hours — because whole days move the day name and leave the clock alone. Say the day out loud at each step; that habit alone fixes most of these.

A flight across time zones is two steps in a fixed order: add the flight time on the departure city's clock, then convert once at the end. Leaving Los Angeles at twenty-three hundred on Saturday and flying twenty hours lands at nineteen hundred on Sunday, LA time; Sydney is seventeen hours ahead, which is Monday midday. You left on Saturday night and arrived on Monday, because the clocks jumped seventeen hours on top of the twenty you spent flying. Going the other way the clock can seem to run backwards, and that is real — check the two steps and trust them.

Four jobs, and a question usually wants two or three of them in order: convert the format, bridge midnight, take off whole days, and apply the time zone once at the end. Saturday oh nine hundred plus seventeen hours is fifteen hours to Sunday midnight and two more, Sunday oh two hundred. Name the day at every step and the hardest item on the paper becomes bookkeeping. Quiz and exercises now.