When you create a task it has a creation date. When I view my Nextcloud tasks in Evolution, it can show creation date. In Thunderbird you can only see the Start Date if it has actually been set which is not the case with most of my tasks.
Creation date should be in the database as indicated by this post:
https://support.mozilla.org/en-US/questions/1128191
I considered making my own add-on adding this feature but there doesn't seem to be any API for tasks.