Hi Kevin.
I open an epub and this is in the opf:
<dc:identifier id="BookID" opf:scheme="UUID">urn:uuid:2c3272a4-731b-4348-909d-6f38b97b18eb</dc:identifier>
That looks pretty meaningless to me, so I opened a second epub opf and this is in there:
<dc:date>2014-07-17</dc:date>
A much more recognisable date code, but it looks more like an "initial creation" date to me than a "last saved" form. In the same opf however, there is this
<dc:identifier id="a9781444792997">urn:uuid:655e14c9-6567-420b-a802-5555d899f567</dc:identifier>
Is that last bit of the "id=" the date 29th Sep 1997 in American format?
So I saved the file to see what changed in Sigil and got this:
<meta name="Sigil version" content="0.9.17" />
<dc:date xmlns
pf="http://www.idpf.org/2007/opf" opf:event="modification">2020-04-21</dc:date>
Much clearer and I suspect that this is what you meant by " date/time metadata is always updated when a svae is done in Sigil. ", but do all/most epub creators update the code on saving? It wasn't something I spotted before, and does it apply to Epub 2 and 3?