They might be subtracting taking first 4 digits and the subtracing yyyy-yyyy, mm-mm and dd-dd.

Which sort of works, but then you also have to deal with the cases where the days or months go negative (at the month and year bounds), and that also involves knowing how many days there are in each month. It's pretty difficult for me to imagine how this could be easier than just converting to e.g. unix time and subtracting seconds.