Date Standards

yyyy/mm/dd, yyyy-mm-dd and yyyymmdd:
differences and when to use each

Dates can be separated by slashes, hyphens, dots or nothing at all. This page explains which to choose for file names, Excel and CSV, including the characters Windows does not allow in file names.

Short answer: choose by purpose. File names: no separator (20260305) or hyphens (2026-03-05). Data exchange between systems: 2026-03-05 (ISO 8601). Documents and screens in Japan: 2026/03/05 is common.

Comparison by separator

StyleExample (March 5, 2026)Typical useWatch out for
yyyy/mm/dd2026/03/05Documents in Japan, Excel displayCannot be used in Windows file names (/ is forbidden)
yyyy-mm-dd2026-03-05Data exchange, APIs, CSV, file namesISO 8601 extended format. Rarely misread
yyyy.mm.dd2026.03.05Internal documents, dot-style companiesConfusing in file names next to the extension dot
yyyy mm dd2026 03 05Specs that show digit groupsOnly when a field asks for it. Unsuitable for file names
yyyymmdd20260305File names, 8-digit system fieldsNo separators; sorts by name = by date

Separators you can and cannot use in file names

Windows does not allow these characters in file names:

\ / : * ? " < > |

So slashes (2026/03/05) and colons (a time such as 14:30) cannot go into a file name. Hyphens (-), underscores (_) and no separator are fine. A dot (.) is allowed but looks like the extension separator, so "2026.03.05_minutes.xlsx" is hard to read.

For file naming in practice, see Standardizing file name dates in yyyyMMdd and Windows file naming best practices.

Excel, CSV and system integration

  • Excel: in a Japanese-language setup, both 2026/03/05 and 2026-03-05 are recognized as dates. The display style is set in Format Cells.
  • CSV: a slash date can be read in a different order depending on the regional settings of the program that opens it. The hyphenated ISO 8601 form is safer.
  • Databases and APIs: most use yyyy-mm-dd (ISO 8601). Follow the specification.

For more on ISO 8601, see What is ISO 8601?

If you cannot decide

For file namesUse yyyymmdd with no separator. Hyphens are fine too, but standardize on one inside your team.
For data exchangeUse yyyy-mm-dd. It is rarely misread across countries and settings.
For screens and documentsyyyy/mm/dd is fine for readers used to it, but do not reuse it in file names.

The idea of standardizing on one format is covered in Standardizing date formats across a team.

Type the same format every time with one key

Changing the separators and digit counts by hand invites mistakes. InsertDate is a Windows app that types today's date in your chosen format when you press the Insert key.

Frequently Asked Questions

What is the difference between yyyy/mm/dd and yyyy-mm-dd?

The order is the same; only the separator differs. The hyphenated yyyy-mm-dd is the ISO 8601 form and suits data exchange. A slash cannot be used in Windows file names.

Which separator should I use in file names?

No separator (yyyymmdd) is the default. If you want one, use a hyphen or an underscore. Slashes and colons cannot be used in file names.

What does yyyy mm dd mean?

It is a way of writing the digit groups for year, month and day separated by spaces. In real input, follow the form: no separator, slashes or hyphens.

Which date format is best for CSV?

yyyy-mm-dd (ISO 8601) is recommended. Slash dates can have month and day swapped depending on the reader's settings.

Try InsertDate Free for 7 Days

Press Insert. Get today's date in yyyyMMdd. Works in almost any Windows app, including Excel, Word and Notepad, with no configuration. To type into an app running as administrator, run InsertDate as administrator too.

Get it on Microsoft Store See features and formats

InsertDate is a Windows 10/11 app. Reading on a phone? Open this page on your PC to install it.