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Created January 18, 2022 01:13
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Falsehoods programmers believe about time, in a single list
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falsehoods-programming-time-list.md

Falsehoods programmers believe about time

This is a compiled list of falsehoods programmers tend to believe about working with time.

Don't re-invent a date time library yourself. If you think you understand everything about time, you're probably doing it wrong.

Falsehoods

  • There are always 24 hours in a day.
  • 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.
  • 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.
  • The offsets between two time zones will remain constant.
  • OK, historical oddities aside, the offsets between two time zones won’t change in the future.
  • Changes in the offsets between time zones will occur with plenty of advance notice.
  • Daylight saving time happens at the same time every year.
  • Daylight saving time happens at the same time in every time zone.
  • Daylight saving time always adjusts by an hour.
  • Months have either 28, 29, 30, or 31 days.
  • The day of the month always advances contiguously from N to either N+1 or 1, with no discontinuities.
  • There is only one calendar system in use at one time.
  • There is a leap year every year divisible by 4.
  • Non leap years will never contain a leap day.
  • It will be easy to calculate the duration of x number of hours and minutes from a particular point in time.
  • The same month has the same number of days in it everywhere!
  • Unix time is completely ignorant about anything except seconds.
  • Unix time is the number of seconds since Jan 1st 1970.
  • The day before Saturday is always Friday.
  • Contiguous timezones are no more than an hour apart. (aka we don’t need to test what happens to the avionics when you fly over the International Date Line)
  • Two timezones that differ will differ by an integer number of half hours.
  • Okay, quarter hours.
  • Okay, seconds, but it will be a consistent difference if we ignore DST.
  • If you create two date objects right beside each other, they’ll represent the same time. (a fantastic Heisenbug generator)
  • You can wait for the clock to reach exactly HH:MM:SS by sampling once a second.
  • If a process runs for n seconds and then terminates, approximately n seconds will have elapsed on the system clock at the time of termination.
  • Weeks start on Monday.
  • Days begin in the morning.
  • Holidays span an integer number of whole days.
  • The weekend consists of Saturday and Sunday.
  • It’s possible to establish a total ordering on timestamps that is useful outside your system.
  • The local time offset (from UTC) will not change during office hours.
  • Thread.sleep(1000) sleeps for 1000 milliseconds.
  • Thread.sleep(1000) sleeps for >= 1000 milliseconds.
  • There are 60 seconds in every minute.
  • Timestamps always advance monotonically.
  • GMT and UTC are the same timezone.
  • Britain uses GMT.
  • Time always goes forwards.
  • The difference between the current time and one week from the current time is always 7 * 86400 seconds.
  • The difference between two timestamps is an accurate measure of the time that elapsed between them.
  • 24:12:34 is a invalid time.
  • Every integer is a theoretical possible year.
  • If you display a datetime, the displayed time has the same second part as the stored time,
  • Or the same year,
  • But at least the numerical difference between the displayed and stored year will be less than 2.
  • If you have a date in a correct YYYY-MM-DD format, the year consists of four characters.
  • If you merge two dates, by taking the month from the first and the day/year from the second, you get a valid date.
  • But it will work, if both years are leap years
  • If you take a w3c published algorithm for adding durations to dates, it will work in all cases.
  • The standard library supports negative years and years above 10000.
  • Time zones always differ by a whole hour.
  • If you convert a timestamp with millisecond precision to a date time with second precision, you can safely ignore the millisecond fractions.
  • But you can ignore the millisecond fraction, if it is less than 0.5.
  • Two-digit years should be somewhere in the range 1900-2099.
  • If you parse a date time, you can read the numbers character for character, without needing to backtrack.
  • But if you print a date time, you can write the numbers character for character, without needing to backtrack.
  • You will never have to parse a format like ---12Z or P12Y34M56DT78H90M12.345S.
  • There are only 24 time zones.
  • Time zones are always whole hours away from UTC.
  • Daylight Saving Time (DST) starts/ends on the same date everywhere.
  • DST is always an advancement by 1 hour.
  • Reading the client’s clock and comparing to UTC is a good way to determine their timezone.
  • The software stack will/won’t try to automatically adjust for timezone/DST.
  • My software is only used internally/locally, so I don’t have to worry about timezones.
  • My software stack will handle it without me needing to do anything special.
  • I can easily maintain a timezone list myself.
  • All measurements of time on a given clock will occur within the same frame of reference.
  • The fact that a date-based function works now means it will work on any date.
  • Years have 365 or 366 days.
  • Each calendar date is followed by the next in sequence, without skipping.
  • A given date and/or time unambiguously identifies a unique moment.
  • Leap years occur every 4 years.
  • You can determine the time zone from the state/province.
  • You can determine the time zone from the city/town.
  • Time passes at the same speed on top of a mountain and at the bottom of a valley.
  • One hour is as long as the next in all time systems.
  • You can calculate when leap seconds will be added.
  • The precision of the data type returned by a getCurrentTime() function is the same as the precision of that function.
  • Two subsequent calls to a getCurrentTime() function will return distinct results.
  • The second of two subsequent calls to a getCurrentTime() function will return a larger result.
  • The software will never run on a space ship that is orbiting a black hole.
  • Devices will be set to the local timezone
  • Users prefer to use the local timezone

Sources

This list is based on these articles. More detailed information about each statement can be found in one of two articles.

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