LCThe Extended Date/Time Format (EDTF) is a specification that addresses a long-standing need in cultural heritage and archival communities: expressing dates that are uncertain, approximate, or only partially known. While ISO 8601 handles precise dates and times well, real-world metadata frequently involves imprecision -- a photograph dated "sometime in the 1940s" or a manuscript created "around 1823." EDTF provides a formal syntax for these cases, enabling both human readability and machine processing.
Background
EDTF was developed by the Library of Congress in collaboration with international partners beginning around 2010. The initial drafts drew on the experience of archivists, librarians, and digital humanists who had long improvised ad hoc notations for uncertain dates. After several years of community review and revision, the EDTF specification was incorporated into the international standard ISO 8601-2:2019, which defines extensions to the base ISO 8601 date/time representation. This gave EDTF formal international standards status while the Library of Congress continues to maintain the specification page and supporting documentation.
Purpose & Scope
EDTF extends ISO 8601 date/time syntax with features for:
- Uncertainty and approximation -- marking a date as uncertain (~), approximate (?), or both (%):
1984?,1984~,1984% - Unspecified digits -- representing partially known dates:
199X,19XX - Intervals -- date ranges with open or unknown endpoints:
1940/1945,../1945 - Sets -- expressing one-of-a-set or all-of-a-set semantics:
[1940, 1942, 1945],{1940..1945} - Seasons -- spring, summer, autumn, winter as date components
- Significant digits -- indicating the precision of a year:
1950S2(precise to the decade)
These features are relevant to metadata in libraries, archives, museums, digital humanities, genealogy, and any domain where date precision varies.
Key Elements / Properties
EDTF is a syntax specification rather than an element set. Its key syntactic features include:
| Feature | Syntax Example | Meaning |
|---|---|---|
| Uncertain | 1984? |
Year is uncertain |
| Approximate | 1984~ |
Year is approximate |
| Uncertain and approximate | 1984% |
Both uncertain and approximate |
| Unspecified | 199X |
Decade known, year not |
| Interval | 2004-02/2005-01 |
February 2004 through January 2005 |
| Open interval | 2004-02/.. |
From February 2004, end unknown |
| Set (one of) | [1940, 1942, 1945] |
One of the listed years |
| Season | 2001-21 |
Spring 2001 |
Serializations & Technical Formats
EDTF dates are encoded as plain text strings. They appear as values within other metadata formats such as Dublin Core, MODS, EAD, and MARC fields. There is no separate serialization -- the string syntax itself is the format. Many programming languages have EDTF parsing libraries available (Python, Ruby, JavaScript, Java).
Governance & Maintenance
The Library of Congress maintains the EDTF specification and its web presence. Since the syntax has been incorporated into ISO 8601-2:2019, formal revisions follow the ISO maintenance process through ISO Technical Committee 154. The LC website remains the primary reference for the specification and implementation guidance.
Notable Implementations
EDTF is supported in a range of library and archival systems. ArchivesSpace uses EDTF for date entry in finding aids. The Samvera/Hyrax digital repository framework supports EDTF date fields. Numerous MODS and Dublin Core implementations accept EDTF-formatted date strings. The Ruby edtf gem, Python edtf-validate library, and JavaScript edtf.js package provide parsing and validation.
Related Standards
- ISO 8601 -- The base international standard for date and time representation. EDTF extends it for imprecise dates.
- Dublin Core -- EDTF strings are commonly used as values in Dublin Core date fields.
- MODS -- The Library of Congress Metadata Object Description Schema, which accepts EDTF date encoding.
- EAD -- Encoded Archival Description, another common consumer of EDTF date values.