ASSYST calculates the due date of next service either on mileage, or distance, whichever is the soonest.
Up until 2006 it was 2 yearly or 15,500 miles. Since then it has been annual, or 15,500 miles. (there is some debate about the exact date of the changeover, and the mileages, but these numbers are illustrative and may be accurate). The date is from date of the pre delivery inspection, at which the ASSYST is activated. Whether yours is a 2 year car, or a one year car would depend on the date of manufacture though.
I guess your car is coming up for an anniversary and ASSYST has flipped to days and is counting down from 30, as it should.