The time
element can’t have a pubdate
attribute. (It was part of early drafts, but it got removed.)
HTML5 itself doesn’t provide a way to specify that a date is a publication or a modification date.
If you need to specify this in a machine-readable way, you could use an appropriate vocabulary. For example, the vocabulary Schema.org defines the properties datePublished
and dateModified
for CreativeWork
(and its sub-types).