Hi all! Jeff from the HubSpot CRM product team here.
We've released a "time between" calculated property type, which makes it possible to calculate the duration between two existing properties. It's now live for all hubs with access to calculated properties, so I'm marketing this as "delivered".
Reading through the comments, there is a lot of interest in being able to calculate the time between the date stored in a property and today's date and time (for tracking time in current stage, or time as a customer, etc.) If you are interested in that functionality, please upvote this community post, and leave a comment with your use case as well.