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Falsehoods programmers believe about time
Over the past couple of years [I have spent a lot of time][checklist] debugging other engineers' test code. This was interesting work, occasi...
# All of these assumptions are wrong
- There are always 24 hours in a day.
- Months have either 30 or 31 days.
- Years have 365 days.
- February is always 28 days long.
- Any 24-hour period will always begin and end in the same day (or week, or month).
- A week always begins and ends in the same month.
- A week (or a month) always begins and ends in the same year.
- The machine that a program runs on will always be in the GMT time zone.
- Ok, that’s not true. But at least the time zone in which a program has to run will never change.
- Well, surely there will never be a change to the time zone in which a program hast to run in production.
- The system clock will always be set to the correct local time.
- The system clock will always be set to a time that is not wildly different from the correct local time.
- If the system clock is incorrect, it will at least always be off by a consistent number of seconds.
- The server clock and the client clock will always be set to the same time.
- The server clock and the client clock will always be set to around the same time.
- Ok, but the time on the server clock and time on the client clock would never be different by a matter of decades.
- If the server clock and the client clock are not in synch, they will at least always be out of synch by a consistent number of seconds.
- The server clock and the client clock will use the same time zone.
-The system clock will never be set to a time that is in the distant past or the far future.
- Time has no beginning and no end.
-One minute on the system clock has exactly the same duration as one minute on any other clock
- Ok, but the duration of one minute on the system clock will be pretty close to the duration of one minute on most other clocks.
- Fine, but the duration of one minute on the system clock would never be more than an hour.
- You can’t be serious.
- The smallest unit of time is one second.
- Ok, one millisecond.
- It will never be necessary to set the system time to any value other than the correct local time.
- Ok, testing might require setting the system time to a value other than the correct local time but it will never be necessary to do so in production.
- Time stamps will always be specified in a commonly-understood format like 1339972628 or 133997262837.
- Time stamps will always be specified in the same format.
- Time stamps will always have the same level of precision.
- A time stamp of sufficient precision can safely be considered unique.
- A timestamp represents the time that an event actually occurred.
- Human-readable dates can be specified in universally understood formats such as 05/07/11.
#programming #date #time
