In Italy we write dates as 14/03/2025, in the United States people write 03/14/2025, and many other countries still have their own variants. As long as the date stays on a sheet of paper read by a local, there is no problem. When it becomes part of a file name, a folder name or a column in a shared document, however, the choice of format has very concrete consequences. The format defined by the international standard ISO 8601, namely YYYY-MM-DD, solves almost all of these problems in one go.
The main advantage: order
Computers sort file names by comparing characters from left to right. With the Italian format, three files dated 05/01/2025, 12/03/2024 and 28/02/2025 are sorted by day:
05-01-2025_minutes.pdf
12-03-2024_minutes.pdf
28-02-2025_minutes.pdf
The order has no chronological meaning. With the ISO format, instead:
2024-03-12_minutes.pdf
2025-01-05_minutes.pdf
2025-02-28_minutes.pdf
Alphabetical order matches chronological order. There is no need to sort by modification date, which changes every time you open and save the file and therefore does not reflect the real date of the document.
No ambiguity
The date 03/04/2025 means 3 April to an Italian and 4 March to an American. If you work with people from other countries, receive documents from foreign suppliers or use programs set to English, this ambiguity can cause serious mistakes, such as a deadline read a month late. A format that starts with a four-digit year leaves no doubt: no widespread system uses year-day-month order.
Useful variants
The standard includes a few forms worth knowing:
| Form | Example | When to use it |
|---|---|---|
| Full date | 2025-03-14 |
almost always |
| Compact form | 20250314 |
when hyphens get in the way, for example in codes |
| Year and month | 2025-03 |
bank statements, monthly reports |
| Date and time | 2025-03-14T09-30 |
several versions on the same day |
| Week | 2025-W11 |
weekly planning |
A note on time: the standard uses a colon between hours and minutes (09:30), but colons are forbidden in file names on Windows. In file names it is therefore best to replace them with a hyphen or drop them: 0930.
The compact form is convenient but less readable at a glance. Personally I prefer the hyphenated form for files that people read and the compact one only where a continuous string is needed.
Where to put the date
In file names the date goes at the beginning when you want documents to sort chronologically, such as minutes, invoices or photographs. It goes after the subject when you want to group first by topic and then by date:
lease-contract_2023-09-01.pdf
electricity-contract_2024-02-15.pdf
Both choices are valid; what matters is following a single rule within the same folder.
Which date to use
A document often has several dates: the issue date, the date you received it, the due date, the date you scanned it. Decide a rule for each type of document and keep to it. For invoices the issue date makes sense; for meeting minutes, the date of the meeting; for a photo, the date it was taken. The save date is almost always the least useful, because the system already records it on its own.
In spreadsheets and documents
The ISO format is useful outside file names too. In a spreadsheet shared with colleagues in other countries, setting the date column to YYYY-MM-DD avoids misunderstandings. Be careful, though: in spreadsheets a date is a numeric value, and the format is only how it is displayed. If you type the date as text, sorting will still work, but calculations between dates will not.
In running text, on the other hand, it remains perfectly correct to write «14 March 2025» in full: it is the most readable form for the reader. The ISO format is an organizational tool, not a stylistic obligation.
Getting used to the format
At first, writing the year first feels unnatural. Two tricks help:
- set your operating system's short date format to YYYY-MM-DD, so you see it everywhere;
- create a text shortcut that inserts today's date in that format.
After a few weeks it becomes automatic, and you realize how awkward the previous system was.
In short
- YYYY-MM-DD makes alphabetical order match chronological order.
- It removes day-month ambiguity with people from other countries.
- In file names, replace the colon in the time with a hyphen.
- Decide which date to use for each document type and where it goes in the name.
- In running text, keep writing dates out in full.
