Another https://ogp.me/#no_vertical object for availability start.
May be distinct from chosen (eg historic) dates shown for news articles.
Values may be reconstructed for existing files from Git history:
perl -i -pE '
close ARGV if eof; # reset line number
$. == 1 or next; # ignore consecutive lines
my $first = qx{git log --follow --pretty=%ai -- "$ARGV" | tail -n1}
or next; # find earliest commit time
chomp $first;
print qq{<meta property="article:published_time" content="$first" />};
say "" unless /^<meta/;
' **/*.html