| Chapter 9: Time |

| 9.9. Comparing and shifting times

Carrying out easy calculations with times is straightforward:

> The chronometer is in the Clock Chamber. "On one wall is a terribly self-important chronometer showing the time in major world cities. London: [time of day]. Paris: [one hour after the time of day]. Tokyo: [9 hours after the time of day]. Cupertino, California: [7 hours before the time of day]."

The phrase "9 hours after the time of day" takes the time of day and adds 9 hours, but this is not the same as ordinary addition of numbers: 9 hours after 11 AM is 8 PM. (This is why we don't use the symbols "+" and "-" when adding times: it would be confusing to the reader.) Similarly, we have conditions

> if the time of day is before 10:45 AM then ...
> Instead of waiting when the time of day is after 4:00 PM: ...

for putting times in order. In recognition of the fact that very few games begin before 4 AM, whereas many run on past midnight, the start of the day is taken to be 4 AM: thus 3:59 AM is after 11:10 PM, but 4:04 AM is before it. Almost all comparisons of time involve the time of day, so we allow the following abbreviations:

> if it is before 10:45 AM then ...
> Instead of waiting when it is after 4:00 PM: ...

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