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xkcd_iso_8601 perfect-date

Nice post about time:


Falsehoods about Time

Some facts about time

  • UTC: The time at zero degrees longitude (the Prime Meridian) is called Universal Coordinated Time (UTC).
  • GMT: UTC used to be called Greenwich Mean Time (GMT) because the Prime Meridian was (arbitrarily) chosen to pass through the Royal Observatory in Greenwich.
  • Other timezones can be written as an offset from UTC. Australian Eastern Standard Time is UTC+1000. e.g. 10:00 UTC is 20:00 EST on the same day.
  • Daylight saving does not affect UTC. It's just a polity deciding to change its timezone (offset from UTC). For example, GMT is still used: it's the British national timezone in winter. In summer it becomes BST.
  • Leap seconds: By international convention, UTC (which is an arbitrary human invention) is kept within 0.9 seconds of physical reality (UT1, which is a measure of solar time) by introducing a "leap second" in the last minute of the UTC year, or in the last minute of June.
  • Leap seconds don't have to be announced much more than six months before they happen. This is a problem if you need second-accurate planning beyond six months.
  • Unix time: Measured as the number of seconds since epoch (the beginning of 1970 in UTC). Unix time is not affected by time zones or daylight saving.
  • According to POSIX.1, Unix time is supposed to handle a leap second by replaying the previous second. e.g.:
59.00 ← replay
00.00 ← increment

This is a trade-off: you can't represent a leap second, and your time is guaranteed to go backwards. On the other hand, every day is exactly 86,400 seconds long, and you don't need a table of all previous and future leap seconds in order to format Unix time as human-preferred hours-minutes-seconds. ntpd is supposed to make the replay happen after it sees the "leap bits" from upstream timeservers, but I've also seen it do nothing: the system goes one second into the future, then slowly slews back to the correct time.

  • Don't blindly use gettimeofday(). If you need a monotonic (always increasing) clock, have a look at clock_gettime().
  • If you want to store a humanly-readable time (e.g. logs), consider storing it along with Unix time, not instead of Unix time.
  • Most of your code shouldn't be dealing with timezones or local time, it should be passing Unix time around.
  • MySQL (at least 4.x and 5.x) stores DATETIME columns as a "YYYY-MM-DD HH:MM:SS" string. I'm not even kidding. If you care at all about storing timestamps, store them as integers and use the UNIX_TIMESTAMP() and FROM_UNIXTIME() functions.
  • The number of "clock" seconds per "real" second is both inaccurate and variable. It mostly varies with temperature.
  • The system clock can, and will, jump backwards and forwards in time due to things outside of your control. Your program should be designed to survive this.
  • The system clock is inaccurate.
  • Time passes at a rate of one second per second for every observer. The frequency of a remote clock relative to an observer is affected by velocity and gravity. The clocks inside GPS satellites are adjusted for relativistic effects.
  • Timezones are a presentation-layer problem!
  • When displaying time, always include the timezone offset. A time format without an offset is useless.
  • When measuring time, measure Unix time. It's UTC. It's easy to obtain. It doesn't have timezone offsets or daylight saving (or leap seconds).
  • When storing time, store Unix time. It's a single number.
  • You're on a network? Every other system's clock is differently inaccurate. ntpd can change the system time in two ways: 1) Step: making the clock jump backwards or forwards to the correct time instantaneously. 2) Slew: changing the frequency of the clock so that it slowly drifts toward the correct time. → Slew is preferred because it's less disruptive, but it's only useful for correcting small offsets.


Falsehoods about Dates

All of these assumptions are wrong:

  • 24:12:34 is an invalid time
  • A given date and/or time unambiguously identifies a unique moment.
  • A time stamp of sufficient precision can safely be considered unique.
  • A timestamp represents the time that an event actually occurred.
  • A week (or a month) always begins and ends in the same year.
  • A week always begins and ends in the same month.
  • All measurements of time on a given clock will occur within the same frame of reference.
  • Any 24-hour period will always begin and end in the same day (or week, or month).
  • Britain uses GMT.
  • But at least the numerical difference between the displayed and stored year will be less than 2
  • But if you print a date time, you can write the numbers character for character, without needing to backtrack
  • But it will work, if both years are leap years
  • But you can ignore the millisecond fraction, if it is less than 0.5
  • Changes in the offsets between time zones will occur with plenty of advance notice.
  • 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)
  • Daylight Saving Time (DST) starts/ends on the same date everywhere
  • Daylight saving time always adjusts by an hour.
  • Daylight saving time happens at the same time every year.
  • Daylight saving time happens at the same time in every time zone.
  • Days begin in the morning.
  • DST is always an advancement by 1 hour
  • Each calendar date is followed by the next in sequence, without skipping.
  • Every integer is a theoretical possible year
  • February is always 28 days long.
  • GMT and UTC are the same timezone.
  • Holidays span an integer number of whole days.
  • Human-readable dates can be specified in universally understood formats such as 05/07/11.
  • I can easily maintain a timezone list myself
  • 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.
  • 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.
  • If the system clock is incorrect, it will at least always be off by a consistent number of seconds.
  • If you convert a timestamp with millisecond precision to a date time with second precision, you can safely ignore the millisecond fractions
  • If you create two date objects right beside each other, they’ll represent the same time. (a fantastic Heisenbug generator)
  • If you display a datetime, the displayed time has the same second part as the stored time
  • 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
  • If you parse a date time, you can read the numbers character for character, without needing to backtrack
  • If you take a w3c published algorithm for adding durations to dates, it will work in all cases.
  • It will be easy to calculate the duration of x number of hours and minutes from a particular point in time.
  • It will never be necessary to set the system time to any value other than the correct local time.
  • It’s possible to establish a total ordering on timestamps that is useful outside your system.
  • Leap years occur every 4 years.
  • Months have either 28, 29, 30, or 31 days.
  • Months have either 30 or 31 days.
  • 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
  • Non leap years will never contain a leap day.
  • OK, historical oddities aside, the offsets between two time zones won’t change in the future.
  • Okay, quarter hours.
  • Okay, seconds, but it will be a consistent difference if we ignore DST.
  • One hour is as long as the next in all time systems.
  • One minute on the system clock has exactly the same duration as one minute on any other clock
  • Or the same year
  • Reading the client’s clock and comparing to UTC is a good way to determine their timezone
  • 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.
  • The day before Saturday is always Friday.
  • The day of the month always advances contiguously from N to either N+1 or 1, with no discontinuities.
  • 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.
  • The duration of one minute on the system clock will be pretty close to the duration of one minute on most other clocks.
  • The duration of one minute on the system clock would never be more than an hour.
  • The fact that a date-based function works now means it will work on any date.
  • The local time offset (from UTC) will not change during office hours.
  • The machine that a program runs on will always be in the GMT time zone.
  • The offsets between two time zones will remain constant.
  • The precision of the data type returned by a getCurrentTime() function is the same as the precision of that function.
  • The same month has the same number of days in it everywhere!
  • The second of two subsequent calls to a getCurrentTime() function will return a larger result.
  • The server clock and the client clock will always be set to around the same time.
  • The server clock and the client clock will always be set to the same time.
  • The server clock and the client clock will use the same time zone.
  • The smallest unit of time is one second.
  • The software stack will/won’t try to automatically adjust for timezone/DST
  • The software will never run on a space ship that is orbiting a black hole.
  • The standard library supports negative years and years above 10000.
  • The system clock will always be set to a time that is not wildly different from the correct local time.
  • The system clock will always be set to the correct local time.
  • The system clock will never be set to a time that is in the distant past or the far future.
  • The time on the server clock and time on the client clock would never be different by a matter of decades.
  • The Time zone in which a program has to run will never change.
  • The weekend consists of Saturday and Sunday.
  • There are 60 seconds in every minute.
  • There are always 24 hours in a day.
  • There are only 24 time zones
  • There is a leap year every year divisible by 4.
  • There is only one calendar system in use at one time.
  • There will never be a change to the time zone in which a program hast to run in production.
  • Thread.sleep(1000) sleeps for >= 1000 milliseconds.
  • Thread.sleep(1000) sleeps for 1000 milliseconds.
  • Time always goes forwards.
  • Time has no beginning and no end.
  • Time passes at the same speed on top of a mountain and at the bottom of a valley.
  • 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.
  • Time zones always differ by a whole hour
  • Time zones are always whole hours away from UTC
  • Timestamps always advance monotonically.
  • Two subsequent calls to a getCurrentTime() function will return distinct results.
  • Two timezones that differ will differ by an integer number of half hours.
  • Two-digit years should be somewhere in the range 1900-2099
  • Unix time is completely ignorant about anything except seconds.
  • Unix time is the number of seconds since Jan 1st 1970.
  • Weeks start on Monday.
  • Years have 365 days.
  • Years have 365 or 366 days.
  • You can calculate when leap seconds will be added.
  • You can determine the time zone from the city/town.
  • You can determine the time zone from the state/province.
  • You can wait for the clock to reach exactly HH:MM:SS by sampling once a second.
  • You will never have to parse a format like ---12Z or P12Y34M56DT78H90M12.345S


Calculate the day of the week

Calculate a number for each of the day, month, and year by adding the digits together as follows:

In this example todays date is 15. June 2023:

Day = just todays day e.g. 15 of June

Month = the number of below table e.g. for June = 3

Value Month
6 Jan
2 Feb
2 Mar
5 Apr
0 May
3 Jun
5 Jul
1 Aug
4 Sep
6 Oct
2 Nov
4 Dec

Years = the number of below table e.g. for 2023 = 0

Value Year
0 00, 06, 17, 23
1 01, 07, 12, 18
2 02, 13, 19, 24
3 03, 08, 14, 25
4 09, 15, 20, 26
5 04, 10, 21, 27
6 05, 11, 16, 22

When calculating the year, either memorize the table or calculate the value using the last two digits e.g. for 2023 = 23. Take 23, divide it by 4, and round down, then add it back to itself. Eg. 23/4 = 5 (rounded down). Adding this to itself we get 28 (23+5). Now, we get the remainder after dividing by 7. In our case of 28, the remainder after division by 7 is 0.

If the year is in the 20th century (1900-1999), add +1. The algorithm will not work for dates before 1582. In our example we add 0. For other centuries use this overview:

Century add this
1500s 1
1600s 0
1700s 5
1800s 3
1900s 1
2000s 0
2100s 5
2200s 3
2300s 1
2400s 0
2500s 5
2600s 3

If the date is January or February of a leap year, then subtract 1 to the final answer. Here is a list of leap years from 1804-2400:

Years Years Years Years Years Years
1804 1904 2004 2104 2204 2304
1808 1908 2008 2108 2208 2308
1812 1912 2012 2112 2212 2312
1816 1916 2016 2116 2216 2316
1820 1920 2020 2120 2220 2320
1824 1924 2024 2124 2224 2324
1828 1928 2028 2128 2228 2328
1832 1932 2032 2132 2232 2332
1836 1936 2036 2136 2236 2336
1840 1940 2040 2140 2240 2340
1844 1944 2044 2144 2244 2344
1848 1948 2048 2148 2248 2348
1852 1952 2052 2152 2252 2352
1856 1956 2056 2156 2256 2356
1860 1960 2060 2160 2260 2360
1864 1964 2064 2164 2264 2364
1868 1968 2068 2168 2268 2368
1872 1972 2072 2172 2272 2372
1876 1976 2076 2176 2276 2376
1880 1980 2080 2180 2280 2380
1884 1984 2084 2184 2284 2384
1888 1988 2088 2188 2288 2388
1892 1992 2092 2192 2292 2392
1896 1996 2096 2196 2296 2396
2000 2400

So finally, in our example we add 15+3+0+0+0 = 18 and get the remainder when dividing the result by 7 = 4.

According to the table below 4 = Thursday so 15. June 2023 was a Thursday :)

Value Day of the week
0 Sunday
1 Monday
2 Tuesday
3 Wednesday
4 Thursday
5 Friday
6 Saturday

The full formula looks like this:

Remainder of (Day + Month(Table) + Year(Table) + Century + (if leap year + 1))/7 = value to look up in day of the week table.


Time zone

Some links to time zone standards:

A list of time zones:

10 Azores Standard Time (GMT-01:00) Azores Atlantic/Azores
12 Cape Verde Standard Time (GMT-01:00) Cape Verde Islands Atlantic/Cape_Verde
43 Mid-Atlantic Standard Time (GMT-02:00) Mid-Atlantic Atlantic/South_Georgia
27 E. South America Standard Time (GMT-03:00) Brasilia America/Sao_Paulo
58 SA Eastern Standard Time (GMT-03:00) Buenos Aires, Georgetown America/Argentina/Buenos_Aires
35 Greenland Standard Time (GMT-03:00) Greenland America/Godthab
51 Newfoundland Standard Time (GMT-03:30) Newfoundland and Labrador America/St_Johns
06 Atlantic Standard Time (GMT-04:00) Atlantic Time (Canada) America/Halifax
60 SA Western Standard Time (GMT-04:00) Caracas, La Paz America/La_Paz
17 Central Brazilian Standard Time (GMT-04:00) Manaus America/Cuiaba
54 Pacific SA Standard Time (GMT-04:00) Santiago America/Santiago
59 SA Pacific Standard Time (GMT-05:00) Bogota, Lima, Quito America/Bogota
28 Eastern Standard Time (GMT-05:00) Eastern Time (US and Canada) America/New_York
70 US Eastern Standard Time (GMT-05:00) Indiana (East) America/Indiana/Indianapolis
15 Central America Standard Time (GMT-06:00) Central America America/Costa_Rica
21 Central Standard Time (GMT-06:00) Central Time (US and Canada) America/Chicago
22 Central Standard Time (Mexico) (GMT-06:00) Guadalajara, Mexico City, Monterrey America/Monterrey
11 Canada Central Standard Time (GMT-06:00) Saskatchewan America/Edmonton
71 US Mountain Standard Time (GMT-07:00) Arizona America/Phoenix
45 Mountain Standard Time (Mexico) (GMT-07:00) Chihuahua, La Paz, Mazatlan America/Chihuahua
44 Mountain Standard Time (GMT-07:00) Mountain Time (US and Canada) America/Denver
55 Pacific Standard Time (GMT-08:00) Pacific Time (US and Canada), Tijuana America/Tijuana
02 Alaskan Standard Time (GMT-09:00) Alaska America/Anchorage
38 Hawaiian Standard Time (GMT-10:00) Hawaii Pacific/Honolulu
61 Samoa Standard Time (GMT-11:00) Midway Island, Samoa Pacific/Apia
36 Greenwich Standard Time (GMT) Casablanca, Monrovia Africa/Monrovia
34 GMT Standard Time (GMT) Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London Europe/London
75 W. Europe Standard Time (GMT+01:00) Amsterdam, Berlin, Bern, Rome, Stockholm, Vienna Europe/Berlin
18 Central Europe Standard Time (GMT+01:00) Belgrade, Bratislava, Budapest, Ljubljana, Prague Europe/Belgrade
56 Romance Standard Time (GMT+01:00) Brussels, Copenhagen, Madrid, Paris Europe/Paris
19 Central European Standard Time (GMT+01:00) Sarajevo, Skopje, Warsaw, Zagreb Europe/Belgrade
74 W. Central Africa Standard Time (GMT+01:00) West Central Africa Africa/Lagos
37 GTB Standard Time (GMT+02:00) Athens, Bucharest, Istanbul Europe/Istanbul
29 Egypt Standard Time (GMT+02:00) Cairo Africa/Cairo
64 South Africa Standard Time (GMT+02:00) Harare, Pretoria Africa/Harare
32 FLE Standard Time (GMT+02:00) Helsinki, Kiev, Riga, Sofia, Tallinn, Vilnius Europe/Riga
41 Israel Standard Time (GMT+02:00) Jerusalem Asia/Jerusalem
26 E. Europe Standard Time (GMT+02:00) Minsk Europe/Minsk
48 Namibia Standard Time (GMT+02:00) Windhoek Africa/Windhoek
05 Arabic Standard Time (GMT+03:00) Baghdad Asia/Baghdad
03 Arab Standard Time (GMT+03:00) Kuwait, Riyadh Asia/Kuwait
57 Russian Standard Time (GMT+03:00) Moscow, St. Petersburg, Volgograd Europe/Moscow
24 E. Africa Standard Time (GMT+03:00) Nairobi Africa/Nairobi
40 Iran Standard Time (GMT+03:30) Tehran Asia/Tehran
04 Arabian Standard Time (GMT+04:00) Abu Dhabi, Muscat Asia/Muscat
09 Azerbaijan Standard Time (GMT+04:00) Baku Asia/Baku
33 Georgian Standard Time (GMT+04:00) Tblisi Asia/Tbilisi
13 Caucasus Standard Time (GMT+04:00) Yerevan Asia/Yerevan
01 Afghanistan Standard Time (GMT+04:30) Kabul Asia/Kabul
30 Ekaterinburg Standard Time (GMT+05:00) Ekaterinburg Asia/Yekaterinburg
76 West Asia Standard Time (GMT+05:00) Islamabad, Karachi, Tashkent Asia/Tashkent
39 India Standard Time (GMT+05:30) Chennai, Kolkata, Mumbai, New Delhi Asia/Calcutta
49 Nepal Standard Time (GMT+05:45) Kathmandu Asia/Kathmandu
47 N. Central Asia Standard Time (GMT+06:00) Almaty, Novosibirsk Asia/Novosibirsk
16 Central Asia Standard Time (GMT+06:00) Astana, Dhaka Asia/Almaty
65 Sri Lanka Standard Time (GMT+06:00) Sri Jayawardenepura Asia/Colombo
46 Myanmar Standard Time (GMT+06:30) Yangon (Rangoon) Asia/Rangoon
62 SE Asia Standard Time (GMT+07:00) Bangkok, Hanoi, Jakarta Asia/Bangkok
53 North Asia Standard Time (GMT+07:00) Krasnoyarsk Asia/Krasnoyarsk
23 China Standard Time (GMT+08:00) Beijing, Chongqing, Hong Kong SAR, Urumqi Asia/Shanghai
52 North Asia East Standard Time (GMT+08:00) Irkutsk, Ulaanbaatar Asia/Irkutsk
63 Singapore Standard Time (GMT+08:00) Kuala Lumpur, Singapore Asia/Singapore
73 W. Australia Standard Time (GMT+08:00) Perth Australia/Perth
66 Taipei Standard Time (GMT+08:00) Taipei Asia/Taipei
68 Tokyo Standard Time (GMT+09:00) Osaka, Sapporo, Tokyo Asia/Tokyo
42 Korea Standard Time (GMT+09:00) Seoul Asia/Seoul
78 Yakutsk Standard Time (GMT+09:00) Yakutsk Asia/Yakutsk
14 Cen. Australia Standard Time (GMT+09:30) Adelaide Australia/Adelaide
07 AUS Central Standard Time (GMT+09:30) Darwin Australia/Darwin
25 E. Australia Standard Time (GMT+10:00) Brisbane Australia/Brisbane
08 AUS Eastern Standard Time (GMT+10:00) Canberra, Melbourne, Sydney Australia/Sydney
77 West Pacific Standard Time (GMT+10:00) Guam, Port Moresby Pacific/Guam
67 Tasmania Standard Time (GMT+10:00) Hobart Australia/Hobart
72 Vladivostok Standard Time (GMT+10:00) Vladivostok Asia/Vladivostok
20 Central Pacific Standard Time (GMT+11:00) Magadan, Solomon Islands, New Caledonia Pacific/Guadalcanal
50 New Zealand Standard Time (GMT+12:00) Auckland, Wellington Pacific/Auckland
31 Fiji Standard Time (GMT+12:00) Fiji Islands, Kamchatka, Marshall Islands Pacific/Fiji
69 Tonga Standard Time (GMT+13:00) Nuku'alofa Pacific/Tongatapu

Lunar Standard Time (LST)

The Lunar year consists of twelve days, named after the first men who walked on the Moon. Each day is divided into 30 cycles of time, with each cycle being divided into 24 moon-hours. Each moon-hour then has 60 moon-minutes, which in turn of course are made up of 60 moon-seconds each. The inverted triangle ∇ is the LST symbol, and if used it suffixes date and prefixes time. The standard notation is: Year-Day-Cycle ∇ Hour:Minute:Second example: 55-11-14 ∇ 14:36:49

Measure Terrestrial Time Lunar Time
seconds 1 0.9843529666671
minute 60 terrestrial seconds 60 lunar seconds
hour 60 terrestrial minutes 60 lunar minutes
day 24 terrestrial hours 30 lunar cycles*
year 365 days 12 lunar days

*Each is day named after one of the twelve men that walked on the Moon during the Apollo projects:

Day Name Days
Day 1 Armstrong 1-30
Day 2 Aldrin 1-30
Day 3 Conrad 1-30
Day 4 Bean 1-30
Day 5 Shepard 1-30
Day 6 Mitchell 1-30
Day 7 Scott 1-30
Day 8 Irwin 1-30
Day 9 Young 1-30
Day 10 Duke 1-30
Day 11 Cernan 1-30
Day 12 Schmitt 1-30

On the moon you have about 15 days of continuous daylight and then 15 days of total darkness. So, a "day" on the Moon, would correspond to about a month or 29.5 Earth-days. Once, about every 29 days, you have a full moon, which is "noon" on the center of the disk. This is also called the synodic month. So a day on the moon, counting from noon to noon, lasts about 29.27 to 29.83 Earth days. Neil Armstrong set foot on the Moon surface on July 21th 1969 at 02:56:15 UT, and this is the obvious choice for a point in time for the calendar to start. So, this is Year 1, day 1 cycle 1, 00:00:00.

Metric Time

Measure Time as we Know it Metric Time Usage
day 24 hours or 1,440 minutes or 86,400 seconds 10 hours or 1,000 minutes or 100,000 seconds
hour 60 minutes or 3,600 seconds 100 minutes or 10,000 seconds
minute 60 seconds 100 seconds



List of calendars:

Representation in various calendars of the year 2021

Calendar Year 2021
Gregorian calendar 2021 MMXXI
Ab urbe condita 2774
Armenian calendar 1470 ԹՎ ՌՆՀ
Assyrian calendar 6771
Bahá'í calendar 177–178
Balinese saka calendar 1942–1943
Bengali calendar 1428
Berber calendar 2971
British Regnal year 69 Eliz. 2 – 70 Eliz. 2
Buddhist calendar 2565
Burmese calendar 1383
Byzantine calendar 7529–7530
Chinese calendar 庚子年 (Metal Rat) 4717 or 4657 — to — 辛丑年 (Metal Ox) 4718 or 4658
Coptic calendar 1737–1738
Discordian calendar 3187
Ethiopian calendar 2013–2014
Hebrew calendar 5781–5782
Hindu calendar Vikram Samvat 2077–2078
Hindu calendar Shaka Samvat 1942–1943
Hindu calendar Kali Yuga 5121–5122
Holocene calendar 12021
Igbo calendar 1021–1022
Iranian calendar 1399–1400
Islamic calendar 1442–1443
Japanese calendar Reiwa 3 (令和3年)
Javanese calendar 1954–1955
Juche calendar 110
Julian calendar Gregorian minus 13 days
Korean calendar 4354
Minguo calendar ROC 110 民國110年
Nanakshahi calendar 553
Thai solar calendar 2564
Tibetan calendar 阳金鼠年 (male Iron-Rat) 2147 or 1766 or 994 — to — 阴金牛年 (female Iron-Ox) 2148 or 1767 or 995
Unix time 1609459200 – 1640995199

Non 7-day weeks

Tradition week length
Bali various
Korea 5 days
Java 5 days
Discordian 5 days
Akan 6 days
Ancient Rome 8 days
Burmese 8 days
Celtic 8 days
Baltic 9 days
Chinese 10 days
Egyptian Calendar 10 days
French Republican Calendar 10 days
Aztecs 13 days


Korean Age vs. International Age

In South Korea, the method of calculating age diverges from the norm observed in most countries. Grasping the distinction between Korean and international age is vital for those engaging with Korean culture. Upon birth, Koreans are already considered a year old, reflecting the time in the womb. Furthermore, every individual's age advances by a year on New Year's Day, irrespective of their actual birth date. Consequently, a Korean's age is consistently one to two years greater than their international counterpart.

The international age system, adopted by the majority of countries, increments a person's age annually on their birthday. By June 2023, to mitigate confusion, the Korean government resolved to exclusively use the international age system for official and legal documentation. This shift means Koreans' official age will now be one to two years less than what's calculated using the traditional Korean method.

In Korean culture, age profoundly influences social dynamics, language nuances, and societal expectations. For example, it's customary for younger individuals to pour drinks, while their elders often cover meal expenses. Inquiring about age in Korea varies based on the individual's age and one's relationship with them, with distinct formal, standard, and informal phrasings. Legal age thresholds, such as for alcohol consumption, align with the international age system. To illustrate, while the Korean age for legal drinking is 20, it translates to 19 in international terms. Though Koreans universally age up on January 1st, personal birthday celebrations occur on the actual date of birth. When two Koreans are of the same age, they refer to their peer relationship as "동갑 (donggap)." This age parity often eases interactions, eliminating the usual age-related social protocols.

Calculating Korean Age:

Calculating Korean age is relatively straightforward. Here's how you can determine someone's Korean age:

  • Start with the Current Year: Begin by taking the current year. For example, if it's 2023, start with that number.
  • Subtract the Birth Year: Deduct the person's birth year from the current year.
  • Add One Year: Since in Korea, a baby is considered one year old at birth (accounting for the time spent in the womb), you add one year to the result from step 2.

For example, if someone was born in 2000 and the current year is 2023:

2023 (current year) - 2000 (birth year) = 23 23 + 1 = 24

So, the person's Korean age in 2023 would be 24.

However, there's one more thing to consider: New Year's Adjustment: Everyone's age in Korea increases by one year on New Year's Day, regardless of their actual birth date. This means if someone's birthday hasn't occurred yet in the current year, their Korean age will be two years more than their international age. Using the example above, if the person's birthday is in July and it's currently February 2023, they would still be 23 in international age but 24 in Korean age.

In summary, the formula is:

Korean Age = (Current Year - Birth Year) + 1

Remember the New Year's adjustment when considering birthdays later in the year.

Fun facts

A moment was a medieval unit of time. The movement of a shadow on a sundial covered 40 moments in a solar hour. An hour in this case meant one twelfth of the period between sunrise and sunset. The length of a solar hour depended on the length of the day, which in turn varied with the season, so the length of a moment in modern seconds was not fixed, but on average, a moment corresponded to 90 seconds.

Fictional calendars

Wikipedia on time, calendars and dates