Your Calendrical Fallacy Is...

False. Even if you live in a place that doesn’t have Daylight Saving Time, you are still subject to rogue leap seconds that get inserted into our calendars every now and then. If you care about being precise, you care about leap seconds. And if you’re writing software for others to use, chances are at least one of your users will be affected by DST at some point.

False. Many places around the world observe Daylight Saving Time, which means that people living in these locations will sometimes experience 23 hour days (when they “leap forward”) and 25 hour days (when they “leap back”).

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?

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.

Mostly true. For the sake of simplifying calculations and establishing conventions, the day-to-day rollover happens at midnight. However, the Hebrew calendar traditionally starts its days at sunset.

False. Many countries (such as most Middle Eastern countries and Israel) use either Thursday and Friday, or Friday and Saturday as their weekend.

False. Weeks start on Sunday in the United States, Monday in Europe, and a couple of places start on Saturday.

False. In Brunei, the work week is Monday - Thursday and Saturday.

False. Many places in Africa use one of the various Coptic calendars. Israel uses the Hebrew calendar for determining holidays. The Islamic calendar is used all through the Middle East and other predominantly Muslim countries. Thailand uses the Buddhist calendar. Japan uses the Japanese Imperial calendar for official state business.

False. February. Duh.

False. The month Pi Kogi Enavot in the Coptic calendar only has 5 or 6 days in it (depending on the year).

Mostly true, but specifically false. Traditional Islamic calendars base their months on local observations of the moon. Sighting the moon determines whether the month will be 29 or 30 days long. However, since you can’t know ahead of time if the moon will be visible (for example: extreme cloud cover), it’s not always possible to know ahead of time how long the month will be.

Currently true, but historically false. A couple of out-of-use calendars, like the Decimal calendar and the Egyptian calendar had weeks that were 7, 8, or even 10 days.

False. It’s the year in the Hebrew calendar.

False. It’s the year in the Japanese calendar.

False. When the “leap forward” transition happens for Daylight Saving Time, an hour is missing. In the US, this is the 2 AM hour, which means that time goes from 1:58 … 1:59 … 3:00 … 3:01 …

False. In 1582, the transition from the Julian to Gregorian calendars meant that there was about a week and a half that was missing for various locations around Europe. That same switch happened in the United States in 1752.

Additionally, in 2011, Samoa decided to jump “ahead” a full day in order to move to the west side of the International Date Line (previously they were on the east). This resulted in December 30, 2011 being skipped on their calendars. So for residents of Samoa, December 29, 2011 was followed by December 31, 2011. The Marshall Islands made the same move in 1993.

False. The Hebrew calendar uses leap months, which occur during the middle of the year.

False. Calendars like the Hebrew and Coptic calendars deal with years that are 13 months long.

False. The Islamic calendar is 354 (or 355) days long.

False. Because the Islamic calendar is only 354 days long, holidays drift by about 10 days each solar year. So after eighteen years, a holiday that used to occur during the winter will be in the summer.

False. The year 1900 was not a leap year.

False. The year 2000 was a leap year.

False. The Hebrew calendar, which is based almost entirely off the lunar cycle, inserts leap years 7 times in a 19 year period.

False. The current year of the Japanese Imperial calendar is based on the reign of the current emperor. When the emperor dies (or abdicates), the next day is reset to the year 1 (of a new era).

For example, the Emperor Akihito abdicated on April 30, Heisei 30 (April 30, 2019). His son, Naruhito, ascended to the throne the next day, which was the date May 1, Reiwa 1.

False. In addition to the Japanese calendar having (potentially) an unbounded number of “New Years” in a single year, the Hebrew calendar historically celebrated up to four different New Years in a single year.

False. Years numbers reset back to 1 when the calendar’s era changes. This is how the Japanese calendar represents a transition from one imperial era to the next (such as Heisei to Reiwa).

False. The Julian calendar is still used by Eastern Orthodox churches, and the Julian day is used in certain kinds of astronomical calculations.

False. Many places have half-hour offsets, such as French Polynesia, Newfoundland and Labrador, Iran, Afghanistan, India, Sri Lanka, Myanmar, the Cocos Islands, North Korea, and parts of Australia.

False. There are a few places that have an offset of :15 of :45 minutes, such as Nepal, part of Western Australia, and the Chatham Islands.

Currently true, but historically false. Before the advent of global communications and standardized time keeping, the current time was pretty much up to the local administrators. When regions were brought in line with the international time keeping standards, they’d have to do some shifts to accomplish that, like Shanghai’s weird 343 second shift in the early 1900s. For more examples, Liberia was 44 minutes behind UTC up until 1972, and from 1880 to 1916, Ireland was 25 minutes and 21 seconds behind UTC.

False. Parts of Oceania are 12:45, 13:00, and 14:00 hours ahead of UTC.

False. As mentioned above, Samoa did just that in 2011. Cancún switched timezones in 2015, as did Turks & Caicos.

False. The Asia/Tomsk timezone was created in 2016 for parts of eastern Russia.

False. Eight different time zones changed their UTC offset in 2016.

False. In 2022, the Europe/Kiev time zone was renamed to Europe/Kyiv.

False. The UNIX epoch is January 1, 1970 in UTC, but is Dec 31, 1969 in Los Angeles.

False. They change multiple times every year.

False. In 2023, Lebanon decided to postpone their DST shift, but then reversed that decision four days later, three days before the shift was scheduled to occur. Egypt did something similar in 2016.

False. Lord Howe Island (part of Australia) observes a 30 minute Daylight Saving Time change. Its standard time zone offset is UTC+10:30, and UTC+11:00 during its summer time.

Historically, the United Kingdom observed “British Double Summer Time” during World War Two, which resulted in a two hour difference from its normal time zone offset. This had the benefit of maximizing available daylight hours during wartime, as well as aligning clocks with their allies and enemies in Western Europe.

False. Because of time zone offsets, a single calendar day typically lasts 50 hours. Or in other words, if you choose a date on the calendar, then there is a 50 hour window where at least some point on the earth thinks it is that day. The day “starts” in the UTC+14 timezone and continues for 14 hours until UTC+0 catches up to that calendar date. That date then continues for 24 hours, at which point UTC+0 transitions out of that timezone, but the other timezones behind UTC are still catching up, all the way to UTC-12.

False. This is very weird. They used to be months 7, 8, 9, and 10, but some reform to the Roman calendar back in the day resulted in the creation of January and February, which messed everything up.

False. There are many weird things about calendars and space travel. Days are longer, days are shorter, time flows faster, time flows slower, days are longer than years, etc etc.

False.

True!

True! You should always use the Date and Time Services provided by the ICU Project. If you’re an iOS/macOS developer, then you should always stick to Calendar and packages based on it, like Time, which are all built on top of the ICU libraries.