Or a timestamp, but that's deprecated. Create it from the last commit, store it in/retrieve it from revision.cc and substitute it in the template. https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-releases
Use the ITS rules file from KDE for itstool: https://websvn.kde.org/trunk/l10n-kf5/scripts/as-metainfo.its?revision=1395611&view=markup