It seems, perhaps that you can selectively export emails, based on https://www.ionos.com/help/email/other-email-programs/mozilla-thunderbird-exporting-emails/. 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.
- Standard Method
- Launch Thunderbird.
- Select your Inbox or another folder.
- Select the email you want to export. Or press CTRL+A to select all emails.
- Click the menu button to display the Thunderbird menu.
- Select Save as > File.
- Select the folder where the emails should be saved and click Save.