For epub3, the full identifier is actually a combination of the book identifier plus the modification date and time. Sigil automatically updates this with every save.
From the epub3 spec ...
Quote:
The Unique Identifier of an EPUB Publication typically SHOULD NOT change with each minor revision to the package or its contents, as Unique Identifiers are intended to have maximal persistence both for referencing and distribution purposes. Each release of an EPUB Publication normally requires that the new version be uniquely identifiable, however, which results in the contradictory need for reliable Unique Identifiers that are changeable.
To redress this problem of identifying minor modifications and releases without changing the Unique Identifier, this specification defines the semantics for a Release Identifier, or means of distinguishing and sequentially ordering EPUB Publications with the same Unique Identifier.
The Release Identifier is not an actual property in the package metadata section, but is a value that can be obtained from two other mandatory pieces of metadata: the Unique Identifier and the last modification date of the Rendition. When the taken together, the combined value represents a unique identity that can be used to distinguish any particular version of an EPUB Publication from another.
To ensure that a Release Identifier can be constructed, each Rendition MUST include exactly one [DCTERMS] modified property containing its last modification date. The value of this property MUST be an [XMLSCHEMA-2] dateTime conformant date of the form:
CCYY-MM-DDThh:mm:ssZ
The last modification date MUST be expressed in Coordinated Universal Time (UTC) and MUST be terminated by the "Z" (Zulu) time zone indicator.
Additional modified properties MAY be included in the package metadata, but they MUST have a different subject (i.e., they require a refines attribute that references an element or resource).
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