Some of the document formats pandoc targets (such as EPUB, docx, and
ODT) include build timestamps in the generated document. That means that
the files generated on successive builds will differ, even if the source
does not. To avoid this, set the SOURCE_DATE_EPOCH
environment variable, and the timestamp will be taken from it instead of
the current time. SOURCE_DATE_EPOCH should contain an
integer unix timestamp (specifying the number of seconds since midnight
UTC January 1, 1970).
Some document formats also include a unique identifier. For EPUB,
this can be set explicitly by setting the identifier
metadata field (see EPUB
Metadata, above).