Talking of the clause earlier mentioned here is an example attached relating to the brittany ferries ^12.50 pp 24 hour return offer.
That's in their general Ts & Cs, but if you look at day trips they have a different set (though some of the wording doesn't reflect that):
Promotional Ferry Booking Terms and Conditions of Carriage
These conditions apply to all ferry bookings with Brittany Ferries. Bookings for ferry travel form a transport contract between yourself and BAI SA, through the agency of its UK▸ subsidiary, BAI UK Limited ('Brittany Ferries'), at the time we confirm your booking by providing your booking reference.
FARES AND GENERAL INFORMATION
Fares are based on travel dates and times and may require you to return to the UK within a specified period. Documents are only valid for the sailing dates/times indicated. Open return tickets are not available. Fares are only valid for itineraries commencing in the country they are booked in. We will not carry passengers under the age of 16 unless an adult of 18 years or over accompanies them. Children travelling with an adult other than, their parents or legal guardians must have a letter of consent.
So where it says "Fares are based on travel dates and times and may require you to return to the UK within a specified period", the conditions for the actual booking will presumably say that with the specific duration or return date specified.
However, I don't think that is made as clear is it ought to be.