ISO 8601 is stricter than many people seem to be aware of. A fair number of posts misunderstand any year-month-day format to be valid.

Brothers and sisters, recall the first commandment: No false gods.

I'll be using the current date and time, May 21, 2024, at 6:04:01 AM, UTC-5, as an example.

Dates

There are two* options:

• 2024-05-21

• 20240521

Impostors abound: 2024/05/21, 2024-5-21, 2024 05 21, 2024 May 21, etc. These are golden cows meant to lead you off the path of righteousness. You must use four-digit years**, two-digit months and days, and delimit with hyphens or nothing.

Times

There are four* options, two with an offset*** and two without:

• T06:04:01.263-05:00

• T060401.263-0500

• T06:04:01.263

• T060401.263

Omitting the offset makes the time ambiguous. It's a good idea to include it if you can.

Times with a positive offset use a plus sign instead of a hyphen-minus, e.g., T14:34:01.263+03:30. For times with no offset (UTC), you can use Z instead of +00:00, e.g., T11:04:01.263Z.

Midnight, 00:00:00, is the start of the day. As of recently, you can use 24:00:00 instead to represent the end of a day. This means that 2024-05-21T24:00:00Z and 2024-05-22T00:00:00Z represent the exact same instant.

You can omit smaller units if you don't need the accuracy. T06:04:01 and T0604 are OK.

You can omit the T if the context makes it unambiguous that it's a time and not a month with no day. (Does 202405 mean May 2024 or 8:24:05 PM?)

Putting it together

You must either…

• use hyphens in the date and colons in the time, or

• use neither.

Again, you have two* options:

• 2024-05-21T06:04:01.263-05:00

• 20240521T060401.263-0500

These are called extended format and basic format, respectively.

Thou shalt not use a space to separate the date and time. (That would be RFC 3339.)

Call to action

This is but the tip of the iceberg. I encourage you to gain a deeper understanding of the Holy Standard and grow in your knowledge of the Good Format by reading the Wikipedia page.

Footnotes

• I'm ignoring less common ISO 8601 formats for simplicity. You can also represent today as 2024-W21-2 or 2024-142, for example. Different denominations, same religion.

** If everyone agrees to a specific higher number of digits, that's allowed with a plus or minus sign. For example, if you agree with me to use seven-year digits, then +0002024-05-21 is valid.

*** Offsets are not the same as time zones. US Central is a time zone. Sometimes it is offset five hours behind UTC; other times it is six hours behind.

I use that system at work. I realized it stays true to ISO 8601, but also eliminate the possible confusion for month/date for people unfamiliar.

Is it standardised somewhere or I just made a hybrid ?