Day field always missing in BibTeX returned from


I am a frequent retriever of BibTeX data via the Crossref API, and I have noticed that no matter for which publications I retrieve metadata, there is never a day field in the returned BibTeX, even if the publication should have a specific publication date specified. Instead, it’s always only “month” and “year”.

As far as I know there is no limitation placed on the format by the creators of BibTeX themselves, and it’s supposed to be able to contain the three of year, month, and day fields, while the successor format, biblatex, is to conform to ISO-8601 and contain only a date field.

Can the day field be added?


Hello, and thanks for your feedback.

Can you point me to some documentation or examples of bibtex that include a day field? I can pass that info along to our metadata retrieval product manager and developer team, if so.

I haven’t been able to find examples from bibtex style guides that include ‘day’. And the original bibtex documentation only states that, if you want to include a day value in the publication date, it should be added along with the month in the ‘month’ field.


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I tried to provide reference URLs in my earlier post, but the discussion board software forbade me from doing so.

I will try again here.

This comment references the differences between the BibTeX and biblatex formats, explaining that while the former only supports year, month, and day fields, the newer latter format can be written using a single ISO-8601-friendly date field. Biblatex is the successor format to BibTeX.

I realize that it day isn’t usually given in the standard list of BibTeX fields, but leaving it out it presents a huge problem. For example, reference/citation software such as Zotero, which rely on imports of *.bibtex files, always have date fields on entires, and the vast majority of modern periodical citations are specified with actual dates of publications, yet *.bibtex files are always missing a date field, and as a result, dates have to be entered manually. It is infuriating, and it’s clear that a date field ought to be in the standard since they included it in its successor format by default.

The official BibTeX specification doesn’t say that including a day field (or other fields) is forbidden, just that it isn’t recognized by standard bibliography styles. However, we don’t only use *.bibtex files for creating styled bibliographies, but also (and increasingly so) for creating databases and records of cited sources, themselves from which such styled bibliographies can be created at a later point (e.g. Zotero and such software). What it says specifically in §3.2 (page 9) is this:

Below is a description of all fields recognized by the standard bibliography styles.
An entry can also contain other fields, which are ignored by those styles.

Adding a day field therefore would allow your API service to be consumed by modern systems that require/can ingest more fields than simply bibliographical styles, and existing bibliography formatters that do not recognize it would simply ignore it.

If you just refuse to add a new field that isn’t typical or standard to such bibliographical styles, then—in order to keep this service and format useful to all applications that consume it—you might consider including the full date, in ISO-8601 format (if it is provided with the original publication) in the note field, which is a “free” field standard to the BibTeX format.