Sorry, nothing to add. I read the post title as "Time Travel" not "Travel Time."
An adventure with time travel, especially one where the DM is running a campaign would be cool. Adventures at 3rd level meet themselves from the future, say 8th level. When they get to 8th level, the players would not be ready for their characters bumping into themselves of the past.
I did something like that with a letter from Ragnar1965's gnome wizard. Sending back in time a warning about what to watch out for because they didn't the first time!
Wow, sorry for the thread derailment.
And now for someone who can answer gmdave's question. Please just move along. Nothing to see in this rambling post.