The Minimum You Need To Know About Timekeeping
A person can spend a lot of time studying how timekeeping works. I enjoy doing that—but you might not, no biggie. However, you still need to understand the basics.
Here’s the minimum of what you need to know, so as not to be terribly wrong anymore:
The entire world’s timekeeping is based on UTC. UTC is a time standard that is kept close to solar time (time measured notionally from the position of the sun in the sky), but whose actual value is maintained through the combined efforts of many atomic clocks worldwide. If you’ve heard about ‘leap seconds’, those are how the two are kept in synch.
GMT is obsolete, and using it for any modern purpose is objectively wrong. GMT is not a synonym for UTC. Longitude 0° doesn’t even pass through Greenwich Observatory in London anymore. The British lie about all this and more due to nationalistic reasons; that doesn’t make them any righter. (For more reading.)
A basic timezone is an offset of UTC (e.g. ‘Pacific Standard Time’ is given by UTC-08:00)—however, almost no one lives in a basic timezone alone. There is usually a second timezone in the same place (e.g. ‘Pacific Daylight Time’, UTC-07:00), and the ‘timezone’ one lives in is a composite (‘Pacific Time’) that switches between standard and daylight time as appropriate. In particular, do not say you live in e.g. ‘Pacific Standard Time’. You live in ‘Pacific Time’.
The de-facto calendar in every country is the International Standard Gregorian Calendar. This includes countries that have different official calendars (those calendars are calculated from the ISO Gregorian Calendar in practice). ISO Gregorian has the familiar months January through December. Years have 365 days, except for leap years with 366, the extra day being added at the end of February. Leap years occur for every year that is divisible by 4, except for multiples of 100 that aren’t also multiples of 400. For example, 2016 and 2020 were leap years. 1700, 1800, and 1900 were not, but 2000 was.
Years are written with four digits and a sign if necessary, never with “AD” / “BC”. E.g. “2025”. Ancient dates are written with a negative sign—e.g. Archimedes was born around the year “-0286”. There is a year zero, written “0000”. You must not use “AD” / “BC” (or various secularizations, such as “BCE”)—those are the specifications for the Traditional Gregorian Calendar, which we do not use anymore because it has various problems. Even historians and religious chronographers reinterpret dates into the ISO calendar—often without even realizing it.
The ‘most correct’ way to write dates is the ISO 8601 format, “YYYY-MM-DD”. E.g. “2025-07-10”. This is logical, consistent, unambiguous, internationally supported, and, being the internal format of computers, the future. Not using this format causes immense confusion and measurable economic damage. You won’t be ‘wrong’ if you use something else, but I will glare haughtily at you if you do.
ISO 8601 also mandates 24-hour time (e.g. “19:00”) instead of “AM” / “PM” (e.g. “7:00 PM”). 24-hour time is already used in most of the world, and it’s clearer and simpler, making adopting it a no-brainer. You should prefer it also.
(Programmers only:) Every computing device (regardless of OS) you use likely keeps time with UNIX seconds, which are not SI seconds. In particular, they will be different at leap seconds in the worst way possible. All timestamps from before the last leap second, if reckoned through UNIX time, the C library, or anything that uses them indirectly, are likely wrong.
As you can see, there’s not a huge amount to know—but all of it is important, and much of it contradicts popular misconceptions you are likely to have internalized. That’s partly why I have created this—so I can link you here, so we can all be just that much more right about such a fundamental topic.
I could, of course, say a lot more about most of these issues—and in some cases I have. For example, if you don’t trust me about the non-existence of GMT, I am pleased to offer far more information and sources than you probably care about. We’re talking the history of Chronometry at the Greenwich Observatory, the cessation of timekeeping operations, the obsolescence per the 3rd General Assembly of the IAU Commission 4 (translated from the original French), and the IERS Reference Meridian shifting due to local gravity anomalies. However, I’m trying to be brief. The above were the high points, and studying these alone will be enough to pass the test of correctness in the wild.
Bluntly, if you disagree with any of the above points, probably I’m right and you’re not. Remember, there is an immense amount of misconception and misinformation out there, to say nothing of outright lies. Sorting through it is a research-level topic. Call me when your relativistically corrected solar system simulator predicts transits of Venus to picosecond accuracy.
Questions and expert corrections are welcome. Thank you for your attention.