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I got it:

post = steem.get_content(post_identifier)
created_timestamp = post.created

For some reason, post.created is not available when you get the post from the stream function:

for post in steem.stream_comments():
created = post.created <---- ERROR

I am not sure if that is a missing feature / bug in piston or just a limitation of the stream function.

@xeroc can you comment?

I have identified the bug .. Will be fixed in the next release or in develop shortly ..
thanks for reporting

Thanks man, you're a legend!