Doesn't count as a rest day because HIIT itself is intense enough to complete as a workout of the day even the one you performed was as short as 12minutes.

Do a tabata/warm-up as short as 4minutes for your rest day if you felt guilt or just want to "feel" a raise in your metabolism. Try not to go for a full HIIT on your rest day. Not saying its not ok, but it just doesn't serve the purpose of having a rest day if you do a HIIT on it.