in reply to Get saved search data out of Thunderbird and into plain-text file

It seems, perhaps that you can selectively export emails, based on I seem to recall I've done this before but for moving to another laptop. It says it's in EML format, which according to the "specification" described in the the EML link, contains data/time info; which is what you're needing. For post processing, I found the reverse of what you might need but it might offer some hints, Mail::Convert::Mbox::ToEml - it's by RJBS so he may also be a good contact. hth