One way to do this would be to put the two dates (quote date and departure date) in the data layer. Then in DTM you could have a page load rule that fires on your quote confirmation page that calculates the days between those two dates and puts it into an evar.
This is exactly what we're doing for this very forum's implementation, actually. We're taking the timestamp of every question, then the timestamp of replies, and bucketing them by day. This allows us to see the amount of time between when a question was asked and when it receives a reply. We then check out the report in Workspace to see each bucket, by day, as dimensions.