This feature has been added to Rate Adjustments, which is found here:
Note: for multiple rate plans on a reservation page, this only works with those plans. If the primary plan attached to a room/unit is different, then on the calendar view, if the rate adjustment doesn't apply to the primary plan, then it won't show. Same for internal calendar. The easy solution is simply to also apply this rate adjustment to the primary rate plan for each room/unit.
Here, we can see I discounted $10 per night for one week from "today" on these places in the system:
Single Unit page
This settings gets sent to Channex, TripAdvisor, and MyAllocator.