Lies programmers believe about calendars

Solid objects turn out to be mostly made of empty space and whirling particles, but we act as though they're solid, because we rarely have to interact with them in such a granular way that involves their underlying complexity.

In the same way, solid iron-clad concepts turn out to be riddled with exceptions that we generally ignore because they're easy to deal with on a case-by-case basis. But when a programmer has to create a system that everyone can use, suddenly these "edge cases" grow to devour the project.

For example: human names are (really) weird. Building a system that can accept all the names people have is really hard. There's actually a giant list of human concepts that are hard to capture in software design.

A worthy addition to that list: Dave Delong's Calendrical Fallacies, AKA lies programmers believe about dates.

An hour will never occur twice in a single day

False. On days when we "leap back" for the Daylight Saving Time shift, one hour occurs twice. For example, in the United States, the hour that occurs twice is the 1 AM hour. This means that on these "fall back" days, correctly-implemented clocks will go from 1:58 … 1:59 … 1:00 … 1:01 … … 1:59 … 2:00 … 2:01 …

This leads to some interesting questions: If a user has set an alarm to wake up at 1 AM on that day, what happens? Does the alarm go off the hour after the midnight hour? Or does it go off during the hour before 2 AM? Or does it go off twice? Or do you just give up and not make the alarm go off at all and make your users miss their dead-of-night appointment?

Every day has a midnight

False. Brazil performs its DST "leap forward" transition at midnight, which means that 11:59 PM is followed by 1:00 AM.

So if you're writing code and are trying to use the time 00:00:00 to represent "no time", you will be wrong in Brazil, and Lebanon in 2017.

Your Calendrical Fallacy Is… [Dave Delong]

(via Four Short Links)