Minor nit. With the Sigil 2.3.1 build with QT 7.6.3, when a missing modified date is added to the opf file, the date format is incorrect. Instead of the terminating Z, it seems to have a +00:00 for the timezone.
Code:
<meta property="dcterms:modified">2024-10-23T19:45:55Z</meta>
<meta property="dcterms:modified">2024-10-30T15:25:44+00:00</meta>