ISO Date Format
ISO 8601 provides an unambiguous calendar-date representation in year-month-day order, commonly shown as YYYY-MM-DD. This is especially useful for data interchange and international technical work, but a human-facing publication can still require a different local or house style.
For ISO-style calendar dates, use year-month-day order such as 2026-09-18; use another presentation only when the governing audience or house style requires it.
Key details
Important caveats
Usage Boundary
Do not describe every numeric date used in ordinary prose as ISO merely because it is unambiguous; apply the standard when the context calls for ISO formatting.
Further guidance
Format
A complete calendar date can be represented in ISO order as YYYY-MM-DD.
Practical Application
Use ISO-style calendar dates in year-month-day order when interoperability, sorting, or a standard-controlled context calls for it. Keep the complete numeric form consistent and do not label an unrelated local date style “ISO.”
Reader Check
Does the context genuinely require ISO formatting, and is the date unambiguous in YYYY-MM-DD order?
Worked Example
“2026-09-28” is an ISO-style calendar date representation; “09/28/26” is not the same format.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- ISO 8601 — Date and time format (opens in a new tab)International Organization for Standardization · ISO overview of unambiguous date/time representation including YYYY-MM-DD