So the code is modifiable in the Blog.RssCode wiki page, see application-blog/application-blog-ui/src/main/resources/Blog/RssCode.xml at c15605fadd4321db2762a83101039fb77fdf777f · xwiki-contrib/application-blog · GitHub
The default formatter is defined at application-blog/application-blog-ui/src/main/resources/Blog/RssCode.xml at c15605fadd4321db2762a83101039fb77fdf777f · xwiki-contrib/application-blog · GitHub & application-blog/application-blog-ui/src/main/resources/Blog/RssCode.xml at c15605fadd4321db2762a83101039fb77fdf777f · xwiki-contrib/application-blog · GitHub
The RSS 1.0 specification doesn’t define a publish date, see RDF Site Summary (RSS) 1.0
However, this was added by the Dublin Core extension, see RDF Site Summary 1.0 Modules: Dublin Core. It points to Date and Time Formats which defines the format for the date/time.
Indeed it does say to use YYYY-MM-DDThh:mm:ssTZD:
hh = two digits of hour (00 through 23) (am/pm NOT allowed)
We use yyyy-MM-dd'T'hh:mm:ssZZ but in Java hh means 0-12 hour format (see Java SimpleDateFormat - Java Date Format | DigitalOcean), and thus it seems that the Blog app is wrong and it should use HH, i.e. yyyy-MM-dd'T'HH:mm:ssZZ
Could you open an issue at Blog Application - XWiki.org JIRA ?
PS: We’ll need to move to RSS 2.0 at some point (which also uses 0-23 hours, see RFC 822 - STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES ). Search for pubDate at RSS 2.0 Specification (Current)