Contributed by Noah Sussman (infinite undo), source

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.

• http://infiniteundo.com/post/25326999628/falsehoods-programmers-believe-about-time
• http://infiniteundo.com/post/25509354022/more-falsehoods-programmers-believe-about-time