Discount for early bird

Hi, I'd like to have a discount for early bird: if a user gets a booking 15 days in advance, he gets a x% reduction.

The existing discount system doesn't allow to work with percentage, which would be very helpful instead of having to do the math.

Also, it would be nice if the discount were being shown on the cart page + email order. Right now they are being calculated but there's no indication that a discount has been made.