No option AFAIK. Basically, you're just copying the *.mbox file from the
archive server to your local machine. If necessary, ImportExportTools
may reformat the data structure of the imported file, but it doesn't
change the data itself. To do so would require that its devs know every
possible variation on how the Date of a post is formatted and inserted
into the header. Those posts are very old. AFAIK, standards back then
were minimal, to put it politely.
I assume you've been looking at the imported *.mbox file within
Thunderbird's /Mail folder. If you can, look at the downloaded file
itself, and check the Date Posted and Date formats in it. That may help
explain why there are two date formats on the messages. But it won't
solve your problem, I'm afraid.
FWIW, I've never been able to figure out the Date thing in Thunderbird.
Your post shows both a Date and an NNTP Posting Date, both different
from the Received date for your message, which is about four hours
before I checked this newsgroup, and is not shown in the headers. AFAIK,
Received is the date it was received by my ISP's server.
Best,
--
Wolf K
kirkwood40.blogspot.com
"The next conference for the time travel design team will be held two
weeks ago."